Description of Business and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation Our Consolidated Financial Statements are prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The Company's Consolidated Financial Statements include the Operating Partnership, wholly owned subsidiaries and those entities in which the Company has the controlling interest. The Operating Partnership's Consolidated Financial Statements include wholly owned subsidiaries and those entities in which the Operating Partnership has the controlling interest. We consolidate joint venture investments, such as interests in partnerships and limited liability companies, when we control the major operating and financial policies of the investment through majority ownership, in our capacity as a general partner or managing member or through some other contractual right. At December 31, 2017, three properties owned through a joint venture investment were consolidated. All intercompany transactions and accounts have been eliminated. During 2015, as a result of our partner’s irrevocable exercise of a buy-sell provision in our SF-HIW Harborview Plaza, LP ("Harborview") joint venture agreement, our partner’s right to put its 80.0% equity interest back to us became no longer exercisable. As a result, we recorded the original contribution transaction as a partial sale. Our investment in this joint venture then qualified for the equity method of accounting, which resulted in the retrospective revision of our Consolidated Statements of Equity and Capital for prior periods.
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 our 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 $184.4 million, $173.1 million and $168.7 million for the years ended December 31, 2017, 2016 and 2015, 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 a 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 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, which consist primarily of compensation, benefits and other costs, such as legal fees related to leasing activities, that are incurred in connection with successfully obtaining leases of properties are also capitalized. 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. The above and below market rate portions of leases acquired in connection with property acquisitions are recorded in deferred 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 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. 1. Description of Business and Significant Accounting Policies – Continued 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 is probable, has been duly approved by the Company, a legally enforceable contract has been executed and the buyer's due diligence period, if any, 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 may impact 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 properties held for use. We record assets held for sale 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 investment, 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 a customer typically pays its pro-rata share of operating and administrative expenses and real estate taxes in excess of the costs incurred during a contractually specified base year. The computation of cost recovery income is complex and involves numerous judgments, including the interpretation of 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 the disposal represents a strategic shift that has (or will have) a major effect on our operations and financial results. Interest expense is included in discontinued operations if a related loan securing the sold property is to be paid off or assumed by the buyer in connection with the sale.
Lease Incentives	Lease Incentives Lease incentive costs, which are payments made to or on behalf of a customer as an incentive to sign a lease, are capitalized in deferred leasing costs and amortized on a straight-line basis over the respective lease terms as a reduction of rental revenues.
Investments in Unconsolidated Affiliates	Investments in Unconsolidated Affiliates We account for our joint venture investments 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 investment. These investments are initially recorded at cost as 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 Equivalents	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.
Redeemable Common Units and Preferred Units	Redeemable Common Units and Preferred Units Limited partners holding Common Units other than the Company (“Redeemable Common Units”) have the right to put any and all of the Common Units to the Operating Partnership and the Company has the right to put any and all of the Preferred Units to the Operating Partnership in exchange for their liquidation preference plus accrued and unpaid distributions in the event of a corresponding redemption by the Company of the underlying Preferred Stock. Consequently, these Redeemable Common Units and Preferred Units are classified outside of permanent partners’ capital in the Operating Partnership's accompanying balance sheets. The recorded value of the Redeemable Common Units is based on fair value at the balance sheet date as measured by the closing price of Common Stock on that date multiplied by the total number of Redeemable Common Units outstanding. The recorded value of the Preferred Units is based on their redemption value.
Income Tax	Income Taxes The Company has elected and expects 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, the Company is required to pay dividends to its stockholders equal to at least 90.0% of its annual REIT taxable income, excluding net capital gains. The partnership agreement requires the Operating Partnership to pay economically equivalent distributions on outstanding Common Units at the same time that the Company pays dividends on its outstanding Common Stock. Other than income taxes related to its taxable REIT subsidiary, the Operating Partnership does not reflect any federal income taxes in its financial statements, since as a partnership the taxable effects of its operations are attributed to its partners. The Operating Partnership does record state income tax for states that tax partnership income directly. 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, 2017, properties that we wholly own were leased to 1,792 customers. The geographic locations that comprise greater than 10.0% of our rental and other revenues are Raleigh, Atlanta, Tampa and Nashville. Our customers engage in a wide variety of businesses. No single customer generated more than 6% of our consolidated revenues during 2017. 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 would subject 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 typically bears interest at fixed rates. Our interest rate risk management objectives are to limit generally the impact of interest rate changes on earnings and cash flows and 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 generally 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. 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 income/(loss) and is subsequently reclassified into interest expense in the period that the hedged forecasted transaction affects earnings. 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 and Per Unit	Earnings Per Share and Per Unit Basic earnings per share of the Company 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 (inclusive of 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 where dividends received on such restricted stock are non-forfeitable. Basic earnings per unit of the Operating Partnership is computed by dividing net income available for common unitholders by the weighted Common Units outstanding - basic. Diluted earnings per unit is computed by dividing net income available to common unitholders by the weighted Common Units outstanding - basic plus the dilutive effect of options and warrants, using the treasury stock method. Weighted Common Units outstanding - basic includes all of the Company's unvested restricted stock where distributions received on such restricted stock are non-forfeitable.
Recently Issued Accounting Standards	Recently Issued Accounting Standards The Financial Accounting Standards Board ("FASB") issued an accounting standards update ("ASU") that requires the use of a new five-step model to recognize revenue from customer contracts. The five-step model requires that we identify the contract with the customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when we satisfy the performance obligations. We will also be required to disclose information regarding the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers. Upon adoption of the ASU in 2018, we expect to utilize the modified retrospective approach which requires a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. We have analyzed the impact of the guidance on our Consolidated Financial Statements, including our internal control processes, and have an active project team continuing to refine our evaluation and finalize our implementation plan with particular emphasis on development of the new required disclosures. Our analysis of our non-lease related revenue contracts, which include primarily real estate sales, management, development and construction fee income and transient parking income, indicates that the adoption of this ASU will require additional financial statement disclosure related to these contracts but will have no material cumulative-effect adjustment or impact on the timing of revenue recognition. There could be additional impact of this ASU upon adoption of the ASU related to accounting for leases discussed below for certain lease revenue streams that may be required to be evaluated as non-lease components using the five-step revenue recognition model. The FASB issued an ASU that adds to and clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows. The ASU is required to be adopted in 2018 with retrospective application required. We do not expect such adoption to have a material effect on our Consolidated Statements of Cash Flows. The FASB issued an ASU that clarifies and narrows the definition of a business used in determining whether to account for a transaction as an asset acquisition or business combination. The guidance requires evaluation of the fair value of the assets acquired to determine if it is concentrated in a single identifiable asset or a group of similar identifiable assets. If so, the transferred assets would not be a business. The guidance also requires a business to include at least one substantive process and narrows the definition of outputs. The ASU is required to be adopted in 2018 and applied prospectively. Upon adoption of this ASU, we expect that the majority of our future acquisitions would not meet the definition of a business; therefore, the related acquisition costs would be capitalized as part of the purchase price. The FASB issued an ASU that clarifies when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. The guidance requires modification accounting if the value, vesting conditions or classification of the award changes. The ASU is required to be adopted in 2018 and applied prospectively. We do not expect such adoption to have a material effect on our Consolidated Financial Statements. The FASB issued an ASU which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both lessees and lessors. We are continuing to conduct our analysis of the impact of the guidance on our Consolidated Financial Statements and have an active project team working on the evaluation and implementation of the guidance. We continue to monitor FASB activity with respect to possible amendments to this ASU, particularly the Board’s recent vote to provide an optional practical expedient to lessors that would remove the requirement for lessors to separate lease and non-lease components when the pattern of recognition of those components are the same and, when combined as a single unit, those would be classified as operating leases. Should such amendment be finalized, we expect to elect the practical expedient. Regardless, we currently believe that the adoption of the ASU will not significantly change the accounting for operating leases on our Consolidated Balance Sheets where we are the lessor, and that such leases will be accounted for in a similar method to existing standards with the underlying leased asset being reported and recognized as a real estate asset. In addition, the guidance requires lessors to capitalize and amortize only incremental direct leasing costs. As a result, we expect that upon the adoption of the ASU, we will no longer be able to capitalize and amortize certain leasing related costs and instead will expense these costs as incurred. We are in the process of evaluating the impact to our results of operations of expensing such costs. The ASU is required to be adopted in 2019 using a modified retrospective approach which requires a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. Our initial analysis of our leases also indicates that upon adoption of the ASU, certain lease revenue streams that are currently accounted for using the lease accounting standard may be accounted for as non-lease components using the five-step revenue recognition model discussed above. Leases where we are the lessee include primarily our operating ground leases which are not material to our Consolidated Financial Statements. We will continue to refine our evaluation and finalize our implementation plan throughout 2018. 1. Description of Business and Significant Accounting Policies – Continued The FASB issued an ASU that eliminates the requirement to separately measure and report hedge ineffectiveness and generally requires the entire change in the fair value of a hedging instrument to be presented in the same income statement line as the hedged item when the hedged item affects earnings. The ASU is required to be adopted in 2019 using a modified retrospective approach. We do not expect such adoption to have a material effect on our Consolidated Financial Statements. The FASB issued an ASU that requires, among other things, the use of a new current expected credit loss ("CECL") model in determining our allowances for doubtful accounts with respect to accounts receivable, accrued straight-line rents receivable and mortgages and notes receivable. The CECL model requires that we estimate our lifetime expected credit loss with respect to these receivables and record allowances that, when deducted from the balance of the receivables, represent the net amounts expected to be collected. We will also be required to disclose information about how we developed the allowances, including changes in the factors (e.g., portfolio mix, credit trends, unemployment, gross domestic product, etc.) that influenced our estimate of expected credit losses and the reasons for those changes. We will apply the ASU’s provisions as a cumulative-effect adjustment to retained earnings upon adoption in 2020. We are in the process of evaluating this ASU.