Description of Business and Significant Accounting Policies Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2012
Description of Business and Significant Accounting Policies [Line Items]
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in accordance with GAAP requires us to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates.
Real Estate and Related Assets	Real Estate and Related Assets Real estate and related assets are recorded at cost and stated at cost less accumulated depreciation. Renovations, replacements and other expenditures that improve or extend the life of assets are capitalized and depreciated over their estimated useful lives. Expenditures for ordinary maintenance and repairs are charged to expense as incurred. Depreciation is computed using the straight-line method over the estimated useful life of 40 years for buildings and depreciable land infrastructure costs, 15 years for building improvements and five to seven years for furniture, fixtures and equipment. Tenant improvements are amortized using the straight-line method over initial fixed terms of the respective leases, which generally are from three to 10 years. Depreciation expense for real estate assets was $129.0 million, $120.8 million and $117.6 million for the years ended December 31, 2012, 2011 and 2010, respectively. Expenditures directly related to the development and construction of real estate assets are included in net real estate assets and are stated at depreciated cost. Development expenditures include pre-construction costs essential to the development of properties, development and construction costs, interest costs on qualifying assets, real estate taxes, development personnel salaries and related costs and other costs incurred during the period of development. Interest and other carrying costs are capitalized until the building is ready for its intended use, but not later than one year from cessation of major construction activity. We consider a construction project as substantially completed and ready for its intended use upon the completion of tenant improvements. We cease capitalization on the portion that is substantially completed and occupied or held available for occupancy, and capitalize only those costs associated with the portion under construction. Expenditures directly related to the leasing of properties are included in deferred financing and leasing costs and are stated at amortized cost. Such expenditures are part of the investment necessary to execute leases and, therefore, are classified as investment activities in the statement of cash flows. All leasing commissions paid to third parties for new leases or lease renewals are capitalized. Internal leasing costs include primarily compensation, benefits and other costs, such as legal fees related to leasing activities, that are incurred in connection with successfully securing leases of properties. Capitalized leasing costs are amortized on a straight-line basis over the initial fixed terms of the respective leases, which generally are from three to 10 years. Estimated costs related to unsuccessful activities are expensed as incurred. We record liabilities for the performance of asset retirement activities when the obligation to perform such activities is probable even when uncertainty exists about the timing and/or method of settlement. Upon the acquisition of real estate assets, we assess the fair value of acquired tangible assets such as land, buildings and tenant improvements, intangible assets and liabilities such as above and below market leases, acquired in-place leases, customer relationships and other identifiable intangible assets and assumed liabilities. We assess fair value based on estimated cash flow projections that utilize discount and/or capitalization rates as well as available market information. The fair value of the tangible assets of an acquired property considers the value of the property as if it were vacant. 1. Description of Business and Significant Accounting Policies – Continued The above and below market rate portions of leases acquired in connection with property acquisitions are recorded in deferred financing and leasing costs and in accounts payable, accrued expenses and other liabilities, respectively, at fair value and amortized into rental revenue over the remaining term of the respective leases as described below. Fair value is calculated as the present value of the difference between (1) the contractual amounts to be paid pursuant to each in-place lease and (2) our estimate of fair market lease rates for each corresponding in-place lease, using a discount rate that reflects the risks associated with the leases acquired and measured over a period equal to the remaining initial term of the lease for above-market leases and the remaining initial term plus the term of any renewal option that the customer would be economically compelled to exercise for below-market leases. In-place leases acquired are recorded at fair value in deferred financing and leasing costs and are amortized to depreciation and amortization expense over the remaining term of the respective lease. The value of in-place leases is based on our evaluation of the specific characteristics of each customer's lease. Factors considered include estimates of carrying costs during hypothetical expected lease-up periods, current market conditions, the customer's credit quality and costs to execute similar leases. In estimating carrying costs, we include real estate taxes, insurance and other operating expenses and estimates of lost rentals at market rates during the expected lease-up periods, depending on local market conditions. In estimating costs to execute similar leases, we consider tenant improvements, leasing commissions and legal and other related expenses. Real estate and other assets are classified as long-lived assets held for use or as long-lived assets held for sale. Real estate is classified as held for sale when the sale of the asset has been duly approved by the Company, a legally enforceable contract has been executed and the buyer's due diligence period has expired.
Impairments of Real Estate Assets and Investments in Unconsolidated Affiliates	Impairments of Real Estate Assets and Investments in Unconsolidated Affiliates With respect to assets classified as held for use, we perform an impairment analysis if events or changes in circumstances indicate that the carrying value may be impaired, such as a significant decline in occupancy, identification of materially adverse legal or environmental factors, change in our designation of an asset from core to non-core which impacts the anticipated holding period or a decline in market value to an amount less than cost. This analysis is generally performed at the property level, except when an asset is part of an interdependent group such as an office park, and consists of determining whether the asset's carrying amount will be recovered from its undiscounted estimated future operating and residual cash flows. These cash flows are estimated based on a number of assumptions that are subject to economic and market uncertainties including, among others, demand for space, competition for customers, changes in market rental rates, costs to operate each property and expected ownership periods. For properties under development, the cash flows are based on expected service potential of the asset or asset group when development is substantially complete. If the carrying amount of a held for use asset exceeds the sum of its undiscounted future operating and residual cash flows, an impairment loss is recorded for the difference between estimated fair value of the asset and the carrying amount. We generally estimate the fair value of assets held for use by using discounted cash flow analyses. In some instances, appraisal information may be available and is used in addition to a discounted cash flow analysis. As the factors used in generating these cash flows are difficult to predict and are subject to future events that may alter our assumptions, the discounted and/or undiscounted future operating and residual cash flows estimated by us in our impairment analyses or those established by appraisal may not be achieved and we may be required to recognize future impairment losses on our properties held for use. We record assets held for sale, including for-sale residential condominiums, at the lower of the carrying amount or estimated fair value. Fair value of assets held for sale is equal to the estimated or contracted sales price with a potential buyer, less costs to sell. The impairment loss is the amount by which the carrying amount exceeds the estimated fair value. We also analyze our investments in unconsolidated affiliates for impairment. This analysis consists of determining whether an expected loss in market value of an investment is other than temporary by evaluating the length of time and the extent to which the market value has been less than cost, the financial condition and near-term prospects of the investee, and our intent and ability to retain our investment for a period of time sufficient to allow for any anticipated recovery in market value. As the factors used in this analysis are difficult to predict and are subject to future events that may alter our assumptions, we may be required to recognize future impairment losses on our investments in unconsolidated affiliates.
Sales of Real Estate	Sales of Real Estate For sales transactions meeting the requirements for full profit recognition, the related assets and liabilities are removed from the balance sheet and the resultant gain or loss is recorded in the period the transaction closes. For sales transactions with continuing involvement after the sale, if the continuing involvement with the property is limited by the terms of the sales contract, profit is recognized at the time of sale and is reduced by the maximum exposure to loss related to the nature of the continuing involvement. Sales to entities in which we have or receive an interest are accounted for using partial sale accounting. For transactions that do not meet the criteria for a sale, we evaluate the nature of the continuing involvement, including put and call provisions, if present, and account for the transaction as a financing arrangement, profit-sharing arrangement, leasing arrangement or other alternate method of accounting, rather than as a sale, based on the nature and extent of the continuing involvement. Some transactions may have numerous forms of continuing involvement. In those cases, we determine which method is most appropriate based on the substance of the transaction.
Rental and Other Revenues	Rental and Other Revenues Minimum contractual rents from leases are recognized on a straight-line basis over the terms of the respective leases. This means that, with respect to a particular lease, actual amounts billed in accordance with the lease during any given period may be higher or lower than the amount of rental revenue recognized for the period. Straight-line rental revenue is commenced when the customer assumes control of the leased premises. Accrued straight-line rents receivable represents the amount by which straight-line rental revenue exceeds rents currently billed in accordance with lease agreements. Contingent rental revenue, such as percentage rent, is accrued when the contingency is removed. Termination fee income is recognized at the later of when the customer has vacated the space or the lease has expired and a fully executed lease termination agreement has been delivered, the amount of the fee is determinable and collectability of the fee is reasonably assured. Rental revenue reductions related to co-tenancy lease provisions, if any, are accrued when events have occurred that trigger such provisions. Cost recovery income is determined on a calendar year and a lease-by-lease basis. The most common types of cost recovery income in our leases are common area maintenance (“CAM”) and real estate taxes, for which the customer pays its pro-rata share of operating and administrative expenses and real estate taxes in excess of a base year amount. The computation of cost recovery income is complex and involves numerous judgments, including the interpretation of terms and other customer lease provisions. Leases are not uniform in dealing with such cost recovery income and there are many variations in the computation. Many customers make monthly fixed payments of CAM, real estate taxes and other cost reimbursement items. We accrue income related to these payments each month. We make quarterly accrual adjustments, positive or negative, to cost recovery income to adjust the recorded amounts to our best estimate of the final annual amounts to be billed and collected. After the end of the calendar year, we compute each customer's final cost recovery income and, after considering amounts paid by the customer during the year, issue a bill or credit for the appropriate amount to the customer. The differences between the amounts billed less previously received payments and the accrual adjustment are recorded as increases or decreases to cost recovery income when the final bills are prepared, which occurs during the first half of the subsequent year.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts Accounts receivable, accrued straight-line rents receivable and mortgages and notes receivable are reduced by an allowance for amounts that may become uncollectible in the future. We regularly evaluate the adequacy of our allowance for doubtful accounts. The evaluation primarily consists of reviewing past due account balances and considering such factors as the credit quality of our customer, historical trends of the customer and changes in customer payment terms. Additionally, with respect to customers in bankruptcy, we estimate the probable recovery through bankruptcy claims and adjust the allowance for amounts deemed uncollectible. If our assumptions regarding the collectability of receivables prove incorrect, we could experience losses in excess of our allowance for doubtful accounts. The allowance and its related receivable are written-off when we have concluded there is a low probability of collection and we have discontinued collection efforts.
Discontinued Operations	Discontinued Operations Properties that are sold or classified as held for sale are classified as discontinued operations provided that (1) the operations and cash flows of the property will be eliminated from our ongoing operations and (2) we will not have any significant continuing involvement in the operations of the property after it is sold. Interest expense is included in discontinued operations if the related loan securing the sold property is to be paid off or assumed by the buyer in connection with the sale. If the property is sold to a joint venture in which we retain an interest, the property will not be accounted for as a discontinued operation due to our significant ongoing interest in the operations through our joint venture interest. If we are retained to provide property management, leasing and/or other services for the property owner after the sale, the property generally will be accounted for as a discontinued operation because the expected cash flows related to our management and leasing activities generally will not be significant in comparison to the cash flows from the property prior to sale.
Lease Incentives	Lease Incentives Lease incentive costs, which are payments made to or on behalf of a customer as an incentive to sign the lease, are capitalized in deferred financing and leasing costs and amortized on a straight-line basis over the respective lease terms as a reduction of rental revenues.
For Sale Residential Condominiums	For-Sale Residential Condominiums For-sale residential condominiums include completed, but unsold, condominium inventory. We initially record receipts of earnest money deposits in accounts payable, accrued expenses and other liabilities in accordance with the deposit method. We then record completed sales when units close and the remaining net cash is received. All for-sale residential condominiums were sold as of December 31, 2012.
Investments in Unconsolidated Affiliates	Investments in Unconsolidated Affiliates We account for our investments in less than majority owned joint ventures, partnerships and limited liability companies using the equity method of accounting when our interests represent a general partnership interest but substantive participating rights or substantive kick out rights have been granted to the limited partners or when our interests do not represent a general partnership interest and we do not control the major operating and financial policies of the entity. These investments are initially recorded at cost in investments in unconsolidated affiliates and are subsequently adjusted for our share of earnings and cash contributions and distributions. To the extent our cost basis at formation of the joint venture is different than the basis reflected at the joint venture level, the basis difference is amortized over the life of the related assets and included in our share of equity in earnings of unconsolidated affiliates.
Cash and Cash Equivalent	Cash Equivalents We consider highly liquid investments with an original maturity of three months or less when purchased to be cash equivalents.
Restricted Cash	Restricted Cash Restricted cash represents cash deposits that are legally restricted or held by third parties on our behalf, such as construction-related escrows, property disposition proceeds set aside and designated or intended to fund future tax-deferred exchanges of qualifying real estate investments, escrows and reserves for debt service, real estate taxes and property insurance established pursuant to certain mortgage financing arrangements and any deposits made with lenders to unencumber secured properties.
Income Tax	Income Taxes We have elected and expect to continue to qualify as a REIT under Sections 856 through 860 of the Internal Revenue Code of 1986, as amended (the “Code”). A corporate REIT is a legal entity that holds real estate assets and, through the payment of dividends to stockholders, is generally permitted to reduce or avoid the payment of federal and state income taxes at the corporate level. To maintain qualification as a REIT, we are required to pay dividends to our stockholders equal to at least 90.0% of our annual REIT taxable income, excluding net capital gains. 1. Description of Business and Significant Accounting Policies – Continued We conduct certain business activities through a taxable REIT subsidiary, as permitted under the Code. The taxable REIT subsidiary is subject to federal, state and local income taxes on its taxable income. We record provisions for income taxes based on its income recognized for financial statement purposes, including the effects of temporary differences between such income and the amount recognized for tax purposes.
Concentration of Credit Risk	Concentration of Credit Risk At December 31, 2012, our Wholly Owned Properties were leased to 1,711 customers. The geographic locations that comprise greater than 10.0% of our rental and other revenues are Raleigh, NC, Atlanta, GA, Tampa, FL, Nashville, TN and Kansas City, MO. Our customers engage in a wide variety of businesses. No single customer of the Wholly Owned Properties generated more than 10.0% of our consolidated revenues during 2012. We maintain our cash and cash equivalents and our restricted cash at financial or other intermediary institutions. The combined account balances at each institution may exceed FDIC insurance coverage and, as a result, there is a concentration of credit risk related to amounts on deposit in excess of FDIC insurance coverage. Additionally, from time to time in connection with tax-deferred 1031 transactions, our restricted cash balances may be commingled with other funds being held by any such intermediary institution which subjects our balance to the credit risk of the institution.
Derivative Financial Instruments	Derivative Financial Instruments We borrow funds at a combination of fixed and variable rates. Borrowings under our revolving credit facility and bank term loans bear interest at variable rates. Our long-term debt, which consists of secured and unsecured long-term financings and the issuance of unsecured debt securities, typically bears interest at fixed rates although some loans bear interest at variable rates. Our interest rate risk management objectives are to limit the impact of interest rate changes on earnings and cash flows and to lower our overall borrowing costs. To achieve these objectives, from time to time, we enter into interest rate hedge contracts such as collars, swaps, caps and treasury lock agreements in order to mitigate our interest rate risk with respect to various debt instruments. We do not hold or issue these derivative contracts for trading or speculative purposes. The interest rate on all of our variable rate debt is generally adjusted at one or three month intervals, subject to settlements under these interest rate hedge contracts. We also enter into treasury lock and similar agreements from time to time in order to limit our exposure to an increase in interest rates with respect to future debt offerings. Interest rate swaps involve the receipt of variable-rate amounts from a counterparty in exchange for making fixed-rate payments over the life of the agreements without exchange of the underlying notional amount. The effective portion of changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in accumulated other comprehensive loss ("AOCL") and is subsequently reclassified into interest expense in the period that the hedged forecasted transaction affects earnings. Interest rate hedge contracts typically contain a provision whereby if we default on any of our indebtedness, we could also be declared in default on our hedge contracts. We account for terminated derivative instruments by recognizing the related accumulated comprehensive income/loss balance in current earnings, unless the hedged forecasted transaction continues as originally planned, in which case we continue to amortize the accumulated comprehensive income/loss into earnings over the originally designated hedge period.
Earnings Per Share	Earnings Per Share Basic earnings per share is computed by dividing net income available for common stockholders by the weighted Common Shares outstanding - basic. Diluted earnings per share is computed by dividing net income available to common stockholders plus noncontrolling interests in the Operating Partnership by the weighted Common Shares outstanding - basic plus the dilutive effect of options, warrants and convertible securities outstanding, including Common Units, using the treasury stock method. Weighted Common Shares outstanding - basic includes all unvested restricted stock since dividends received on such restricted stock are non-forfeitable.

Description of Business and Significant Accounting Policies Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2012

Description of Business and Significant Accounting Policies [Line Items]

The preparation of consolidated financial statements in accordance with GAAP requires us to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from those estimates.

Real Estate and Related Assets

Real estate and related assets are recorded at cost and stated at cost less accumulated depreciation. Renovations, replacements and other expenditures that improve or extend the life of assets are capitalized and depreciated over their estimated useful lives. Expenditures for ordinary maintenance and repairs are charged to expense as incurred. Depreciation is computed using the straight-line method over the estimated useful life of 40 years for buildings and depreciable land infrastructure costs, 15 years for building improvements and five to seven years for furniture, fixtures and equipment. Tenant improvements are amortized using the straight-line method over initial fixed terms of the respective leases, which generally are from three to 10 years. Depreciation expense for real estate assets was $129.0 million, $120.8 million and $117.6 million for the years ended December 31, 2012, 2011 and 2010, respectively.

Expenditures directly related to the development and construction of real estate assets are included in net real estate assets and are stated at depreciated cost. Development expenditures include pre-construction costs essential to the development of properties, development and construction costs, interest costs on qualifying assets, real estate taxes, development personnel salaries and related costs and other costs incurred during the period of development. Interest and other carrying costs are capitalized until the building is ready for its intended use, but not later than one year from cessation of major construction activity. We consider a construction project as substantially completed and ready for its intended use upon the completion of tenant improvements. We cease capitalization on the portion that is substantially completed and occupied or held available for occupancy, and capitalize only those costs associated with the portion under construction.

Expenditures directly related to the leasing of properties are included in deferred financing and leasing costs and are stated at amortized cost. Such expenditures are part of the investment necessary to execute leases and, therefore, are classified as investment activities in the statement of cash flows. All leasing commissions paid to third parties for new leases or lease renewals are capitalized. Internal leasing costs include primarily compensation, benefits and other costs, such as legal fees related to leasing activities, that are incurred in connection with successfully securing leases of properties. Capitalized leasing costs are amortized on a straight-line basis over the initial fixed terms of the respective leases, which generally are from three to 10 years. Estimated costs related to unsuccessful activities are expensed as incurred.

We record liabilities for the performance of asset retirement activities when the obligation to perform such activities is probable even when uncertainty exists about the timing and/or method of settlement.

Upon the acquisition of real estate assets, we assess the fair value of acquired tangible assets such as land, buildings and tenant improvements, intangible assets and liabilities such as above and below market leases, acquired in-place leases, customer relationships and other identifiable intangible assets and assumed liabilities. We assess fair value based on estimated cash flow projections that utilize discount and/or capitalization rates as well as available market information. The fair value of the tangible assets of an acquired property considers the value of the property as if it were vacant.

1. Description of Business and Significant Accounting Policies – Continued

The above and below market rate portions of leases acquired in connection with property acquisitions are recorded in deferred financing and leasing costs and in accounts payable, accrued expenses and other liabilities, respectively, at fair value and amortized into rental revenue over the remaining term of the respective leases as described below. Fair value is calculated as the present value of the difference between (1) the contractual amounts to be paid pursuant to each in-place lease and (2) our estimate of fair market lease rates for each corresponding in-place lease, using a discount rate that reflects the risks associated with the leases acquired and measured over a period equal to the remaining initial term of the lease for above-market leases and the remaining initial term plus the term of any renewal option that the customer would be economically compelled to exercise for below-market leases.

In-place leases acquired are recorded at fair value in deferred financing and leasing costs and are amortized to depreciation and amortization expense over the remaining term of the respective lease. The value of in-place leases is based on our evaluation of the specific characteristics of each customer's lease. Factors considered include estimates of carrying costs during hypothetical expected lease-up periods, current market conditions, the customer's credit quality and costs to execute similar leases. In estimating carrying costs, we include real estate taxes, insurance and other operating expenses and estimates of lost rentals at market rates during the expected lease-up periods, depending on local market conditions. In estimating costs to execute similar leases, we consider tenant improvements, leasing commissions and legal and other related expenses.

Real estate and other assets are classified as long-lived assets held for use or as long-lived assets held for sale. Real estate is classified as held for sale when the sale of the asset has been duly approved by the Company, a legally enforceable contract has been executed and the buyer's due diligence period has expired.

Impairments of Real Estate Assets and Investments in Unconsolidated Affiliates

With respect to assets classified as held for use, we perform an impairment analysis if events or changes in circumstances indicate that the carrying value may be impaired, such as a significant decline in occupancy, identification of materially adverse legal or environmental factors, change in our designation of an asset from core to non-core which impacts the anticipated holding period or a decline in market value to an amount less than cost. This analysis is generally performed at the property level, except when an asset is part of an interdependent group such as an office park, and consists of determining whether the asset's carrying amount will be recovered from its undiscounted estimated future operating and residual cash flows. These cash flows are estimated based on a number of assumptions that are subject to economic and market uncertainties including, among others, demand for space, competition for customers, changes in market rental rates, costs to operate each property and expected ownership periods. For properties under development, the cash flows are based on expected service potential of the asset or asset group when development is substantially complete.

If the carrying amount of a held for use asset exceeds the sum of its undiscounted future operating and residual cash flows, an impairment loss is recorded for the difference between estimated fair value of the asset and the carrying amount. We generally estimate the fair value of assets held for use by using discounted cash flow analyses. In some instances, appraisal information may be available and is used in addition to a discounted cash flow analysis. As the factors used in generating these cash flows are difficult to predict and are subject to future events that may alter our assumptions, the discounted and/or undiscounted future operating and residual cash flows estimated by us in our impairment analyses or those established by appraisal may not be achieved and we may be required to recognize future impairment losses on our properties held for use.

We record assets held for sale, including for-sale residential condominiums, at the lower of the carrying amount or estimated fair value. Fair value of assets held for sale is equal to the estimated or contracted sales price with a potential buyer, less costs to sell. The impairment loss is the amount by which the carrying amount exceeds the estimated fair value.

We also analyze our investments in unconsolidated affiliates for impairment. This analysis consists of determining whether an expected loss in market value of an investment is other than temporary by evaluating the length of time and the extent to which the market value has been less than cost, the financial condition and near-term prospects of the investee, and our intent and ability to retain our investment for a period of time sufficient to allow for any anticipated recovery in market value. As the factors used in this analysis are difficult to predict and are subject to future events that may alter our assumptions, we may be required to recognize future impairment losses on our investments in unconsolidated affiliates.

Sales of Real Estate

For sales transactions meeting the requirements for full profit recognition, the related assets and liabilities are removed from the balance sheet and the resultant gain or loss is recorded in the period the transaction closes. For sales transactions with continuing involvement after the sale, if the continuing involvement with the property is limited by the terms of the sales contract, profit is recognized at the time of sale and is reduced by the maximum exposure to loss related to the nature of the continuing involvement. Sales to entities in which we have or receive an interest are accounted for using partial sale accounting.

For transactions that do not meet the criteria for a sale, we evaluate the nature of the continuing involvement, including put and call provisions, if present, and account for the transaction as a financing arrangement, profit-sharing arrangement, leasing arrangement or other alternate method of accounting, rather than as a sale, based on the nature and extent of the continuing involvement. Some transactions may have numerous forms of continuing involvement. In those cases, we determine which method is most appropriate based on the substance of the transaction.

Rental and Other Revenues

Minimum contractual rents from leases are recognized on a straight-line basis over the terms of the respective leases. This means that, with respect to a particular lease, actual amounts billed in accordance with the lease during any given period may be higher or lower than the amount of rental revenue recognized for the period. Straight-line rental revenue is commenced when the customer assumes control of the leased premises. Accrued straight-line rents receivable represents the amount by which straight-line rental revenue exceeds rents currently billed in accordance with lease agreements. Contingent rental revenue, such as percentage rent, is accrued when the contingency is removed. Termination fee income is recognized at the later of when the customer has vacated the space or the lease has expired and a fully executed lease termination agreement has been delivered, the amount of the fee is determinable and collectability of the fee is reasonably assured. Rental revenue reductions related to co-tenancy lease provisions, if any, are accrued when events have occurred that trigger such provisions.

Cost recovery income is determined on a calendar year and a lease-by-lease basis. The most common types of cost recovery income in our leases are common area maintenance (“CAM”) and real estate taxes, for which the customer pays its pro-rata share of operating and administrative expenses and real estate taxes in excess of a base year amount. The computation of cost recovery income is complex and involves numerous judgments, including the interpretation of terms and other customer lease provisions. Leases are not uniform in dealing with such cost recovery income and there are many variations in the computation. Many customers make monthly fixed payments of CAM, real estate taxes and other cost reimbursement items. We accrue income related to these payments each month. We make quarterly accrual adjustments, positive or negative, to cost recovery income to adjust the recorded amounts to our best estimate of the final annual amounts to be billed and collected. After the end of the calendar year, we compute each customer's final cost recovery income and, after considering amounts paid by the customer during the year, issue a bill or credit for the appropriate amount to the customer. The differences between the amounts billed less previously received payments and the accrual adjustment are recorded as increases or decreases to cost recovery income when the final bills are prepared, which occurs during the first half of the subsequent year.

Allowance for Doubtful Accounts

Accounts receivable, accrued straight-line rents receivable and mortgages and notes receivable are reduced by an allowance for amounts that may become uncollectible in the future. We regularly evaluate the adequacy of our allowance for doubtful accounts. The evaluation primarily consists of reviewing past due account balances and considering such factors as the credit quality of our customer, historical trends of the customer and changes in customer payment terms. Additionally, with respect to customers in bankruptcy, we estimate the probable recovery through bankruptcy claims and adjust the allowance for amounts deemed uncollectible. If our assumptions regarding the collectability of receivables prove incorrect, we could experience losses in excess of our allowance for doubtful accounts. The allowance and its related receivable are written-off when we have concluded there is a low probability of collection and we have discontinued collection efforts.

Discontinued Operations

Properties that are sold or classified as held for sale are classified as discontinued operations provided that (1) the operations and cash flows of the property will be eliminated from our ongoing operations and (2) we will not have any significant continuing involvement in the operations of the property after it is sold. Interest expense is included in discontinued operations if the related loan securing the sold property is to be paid off or assumed by the buyer in connection with the sale. If the property is sold to a joint venture in which we retain an interest, the property will not be accounted for as a discontinued operation due to our significant ongoing interest in the operations through our joint venture interest. If we are retained to provide property management, leasing and/or other services for the property owner after the sale, the property generally will be accounted for as a discontinued operation because the expected cash flows related to our management and leasing activities generally will not be significant in comparison to the cash flows from the property prior to sale.

Lease Incentives

Lease incentive costs, which are payments made to or on behalf of a customer as an incentive to sign the lease, are capitalized in deferred financing and leasing costs and amortized on a straight-line basis over the respective lease terms as a reduction of rental revenues.

For Sale Residential Condominiums

For-Sale Residential Condominiums

For-sale residential condominiums include completed, but unsold, condominium inventory. We initially record receipts of earnest money deposits in accounts payable, accrued expenses and other liabilities in accordance with the deposit method. We then record completed sales when units close and the remaining net cash is received. All for-sale residential condominiums were sold as of December 31, 2012.

Investments in Unconsolidated Affiliates

We account for our investments in less than majority owned joint ventures, partnerships and limited liability companies using the equity method of accounting when our interests represent a general partnership interest but substantive participating rights or substantive kick out rights have been granted to the limited partners or when our interests do not represent a general partnership interest and we do not control the major operating and financial policies of the entity. These investments are initially recorded at cost in investments in unconsolidated affiliates and are subsequently adjusted for our share of earnings and cash contributions and distributions. To the extent our cost basis at formation of the joint venture is different than the basis reflected at the joint venture level, the basis difference is amortized over the life of the related assets and included in our share of equity in earnings of unconsolidated affiliates.

Cash and Cash Equivalent

Cash Equivalents

We consider highly liquid investments with an original maturity of three months or less when purchased to be cash equivalents.

Restricted Cash

Restricted cash represents cash deposits that are legally restricted or held by third parties on our behalf, such as construction-related escrows, property disposition proceeds set aside and designated or intended to fund future tax-deferred exchanges of qualifying real estate investments, escrows and reserves for debt service, real estate taxes and property insurance established pursuant to certain mortgage financing arrangements and any deposits made with lenders to unencumber secured properties.

Income Tax

Income Taxes

We have elected and expect to continue to qualify as a REIT under Sections 856 through 860 of the Internal Revenue Code of 1986, as amended (the “Code”). A corporate REIT is a legal entity that holds real estate assets and, through the payment of dividends to stockholders, is generally permitted to reduce or avoid the payment of federal and state income taxes at the corporate level. To maintain qualification as a REIT, we are required to pay dividends to our stockholders equal to at least 90.0% of our annual REIT taxable income, excluding net capital gains.

1. Description of Business and Significant Accounting Policies – Continued

We conduct certain business activities through a taxable REIT subsidiary, as permitted under the Code. The taxable REIT subsidiary is subject to federal, state and local income taxes on its taxable income. We record provisions for income taxes based on its income recognized for financial statement purposes, including the effects of temporary differences between such income and the amount recognized for tax purposes.

Concentration of Credit Risk

At December 31, 2012, our Wholly Owned Properties were leased to 1,711 customers. The geographic locations that comprise greater than 10.0% of our rental and other revenues are Raleigh, NC, Atlanta, GA, Tampa, FL, Nashville, TN and Kansas City, MO. Our customers engage in a wide variety of businesses. No single customer of the Wholly Owned Properties generated more than 10.0% of our consolidated revenues during 2012.

We maintain our cash and cash equivalents and our restricted cash at financial or other intermediary institutions. The combined account balances at each institution may exceed FDIC insurance coverage and, as a result, there is a concentration of credit risk related to amounts on deposit in excess of FDIC insurance coverage. Additionally, from time to time in connection with tax-deferred 1031 transactions, our restricted cash balances may be commingled with other funds being held by any such intermediary institution which subjects our balance to the credit risk of the institution.

Derivative Financial Instruments

We borrow funds at a combination of fixed and variable rates. Borrowings under our revolving credit facility and bank term loans bear interest at variable rates. Our long-term debt, which consists of secured and unsecured long-term financings and the issuance of unsecured debt securities, typically bears interest at fixed rates although some loans bear interest at variable rates. Our interest rate risk management objectives are to limit the impact of interest rate changes on earnings and cash flows and to lower our overall borrowing costs. To achieve these objectives, from time to time, we enter into interest rate hedge contracts such as collars, swaps, caps and treasury lock agreements in order to mitigate our interest rate risk with respect to various debt instruments. We do not hold or issue these derivative contracts for trading or speculative purposes. The interest rate on all of our variable rate debt is generally adjusted at one or three month intervals, subject to settlements under these interest rate hedge contracts. We also enter into treasury lock and similar agreements from time to time in order to limit our exposure to an increase in interest rates with respect to future debt offerings.

Interest rate swaps involve the receipt of variable-rate amounts from a counterparty in exchange for making fixed-rate payments over the life of the agreements without exchange of the underlying notional amount. The effective portion of changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in accumulated other comprehensive loss ("AOCL") and is subsequently reclassified into interest expense in the period that the hedged forecasted transaction affects earnings. Interest rate hedge contracts typically contain a provision whereby if we default on any of our indebtedness, we could also be declared in default on our hedge contracts.

We account for terminated derivative instruments by recognizing the related accumulated comprehensive income/loss balance in current earnings, unless the hedged forecasted transaction continues as originally planned, in which case we continue to amortize the accumulated comprehensive income/loss into earnings over the originally designated hedge period.

Earnings Per Share

Basic earnings per share is computed by dividing net income available for common stockholders by the weighted Common Shares outstanding - basic. Diluted earnings per share is computed by dividing net income available to common stockholders plus noncontrolling interests in the Operating Partnership by the weighted Common Shares outstanding - basic plus the dilutive effect of options, warrants and convertible securities outstanding, including Common Units, using the treasury stock method. Weighted Common Shares outstanding - basic includes all unvested restricted stock since dividends received on such restricted stock are non-forfeitable.