Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Document Fiscal Year Focus	2015
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation, net of capitalization	$ 2,900	$ 2,272
Cash flows from financing activities:
Net proceeds from common stock issuance	1,173
Parent Company [Member]
Incremental Common Shares Attributable to Dilutive Effect of Equity Forward Agreements	7	0
Cash flows from operating activities:
Net income	30,992	25,056
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	35,992	37,905
Amortization of deferred loan cost and debt premium	2,447	2,889
Amortization and (accretion) of above and below market lease intangibles, net	(521)	(861)
Stock-based compensation, net of capitalization	2,900	2,272
Equity in income of investments in real estate partnerships	(5,567)	(7,808)
Gain on sale of real estate	(803)	(715)
Provision for impairment	0	(225)
Early extinguishment of debt	61	0
Distribution of earnings from operations of investments in real estate partnerships	(12,301)	(8,681)
Gain on derivative instruments	0	(5)
Deferred compensation expense	631	200
Realized and unrealized gain on investments	(634)	(201)
Changes in assets and liabilities:
Restricted cash	1,415	328
Accounts receivable	(7,827)	(6,740)
Straight-line rent receivables, net	(1,877)	(1,594)
Deferred leasing costs	(1,816)	(1,961)
Other assets	(1,137)	(1,952)
Accounts payable and other liabilities	(13,101)	(4,858)
Tenants’ security and escrow deposits and prepaid rent	1,124	1,403
Net cash provided by operating activities	54,458	52,264
Cash flows from investing activities:
Acquisition of operating real estate	0	(78,943)
Proceeds from Deposits on Real Estate Sales	(4,000)	0
Real estate development and capital improvements	(55,047)	(46,648)
Proceeds from sale of real estate investments	3,414	4,472
Investments in real estate partnerships	(1,344)	(1,771)
Distributions received from investments in real estate partnerships	2,717	5,931
Dividends on investments	31	27
Acquisition of securities	(3,726)	(1,797)
Proceeds from sale of securities	2,868	1,676
Net cash used in investing activities	(55,087)	(117,053)
Cash flows from financing activities:
Net proceeds from common stock issuance	992	0
Redemption of preferred stock and partnership units	0	(300)
Distributions to limited partners in consolidated partnerships, net	(1,050)	(426)
Distributions to exchangeable operating partnership unit holders	(76)	(78)
Dividends paid to common stockholders	(45,273)	(42,947)
Dividends paid to preferred stockholders	(5,266)	(5,266)
Proceeds from unsecured credit facilities	40,000	70,000
Repayment of unsecured credit facilities	(10,000)	0
Proceeds from notes payable	1,351	0
Repayments of Notes Payable	55,777	0
Scheduled principal payments	(1,481)	(1,719)
Payment of loan costs	(78)	0
Net cash (used in) provided by financing activities	(76,658)	19,264
Net decrease in cash and cash equivalents	(77,287)	(45,525)
Cash and cash equivalents at beginning of the period	113,776	80,684
Cash and cash equivalents at end of the period	36,489	35,159
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $2,059 and $1,641 in 2015 and 2014, respectively)	17,964	18,338
Income Taxes Paid	697	0
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed for the acquisition of real estate	0	78,049
Change in fair value of derivative instruments	(13,882)	(13,800)
Common stock issued for dividend reinvestment plan	374	313
Stock-based compensation capitalized	701	696
Contributions from limited partners in consolidated partnerships, net	13	58
Non-controlling interest recorded, fair value	0	15,385
Common stock issued for dividend reinvestment in trust	214	189
Contribution of stock awards into trust	1,042	1,440
Distribution of stock held in trust	0	4
Partnership Interest [Member]
Incremental Common Shares Attributable to Dilutive Effect of Equity Forward Agreements	7	0
Cash flows from operating activities:
Net income	30,992	25,056
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	35,992	37,905
Amortization of deferred loan cost and debt premium	2,447	2,889
Amortization and (accretion) of above and below market lease intangibles, net	(521)	(861)
Stock-based compensation, net of capitalization	2,900	2,272
Equity in income of investments in real estate partnerships	(5,567)	(7,808)
Gain on sale of real estate	(803)	(715)
Provision for impairment	0	(225)
Early extinguishment of debt	61	0
Distribution of earnings from operations of investments in real estate partnerships	(12,301)	(8,681)
Gain on derivative instruments	0	(5)
Deferred compensation expense	631	200
Realized and unrealized gain on investments	(634)	(201)
Changes in assets and liabilities:
Restricted cash	1,415	328
Accounts receivable	(7,827)	(6,740)
Straight-line rent receivables, net	(1,877)	(1,594)
Deferred leasing costs	(1,816)	(1,961)
Other assets	(1,137)	(1,952)
Accounts payable and other liabilities	(13,101)	(4,858)
Tenants’ security and escrow deposits and prepaid rent	1,124	1,403
Net cash provided by operating activities	54,458	52,264
Cash flows from investing activities:
Acquisition of operating real estate	0	(78,943)
Proceeds from Deposits on Real Estate Sales	(4,000)	0
Real estate development and capital improvements	(55,047)	(46,648)
Proceeds from sale of real estate investments	3,414	4,472
Investments in real estate partnerships	(1,344)	(1,771)
Distributions received from investments in real estate partnerships	2,717	5,931
Dividends on investments	31	27
Acquisition of securities	(3,726)	(1,797)
Proceeds from sale of securities	2,868	1,676
Net cash used in investing activities	(55,087)	(117,053)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	992	0
Redemption of preferred stock and partnership units	0	(300)
Distributions to limited partners in consolidated partnerships, net	(1,050)	(426)
Dividends paid to common stockholders	(45,349)	(43,025)
Dividends paid to preferred stockholders	(5,266)	(5,266)
Proceeds from unsecured credit facilities	40,000	70,000
Repayment of unsecured credit facilities	(10,000)	0
Proceeds from notes payable	1,351	0
Repayments of Notes Payable	55,777	0
Scheduled principal payments	(1,481)	(1,719)
Payment of loan costs	(78)	0
Net cash (used in) provided by financing activities	(76,658)	19,264
Net decrease in cash and cash equivalents	(77,287)	(45,525)
Cash and cash equivalents at beginning of the period	113,776	80,684
Cash and cash equivalents at end of the period	36,489	35,159
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $2,059 and $1,641 in 2015 and 2014, respectively)	17,964	18,338
Income Taxes Paid	697	0
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed for the acquisition of real estate	0	78,049
Change in fair value of derivative instruments	(13,882)	(13,800)
Common stock issued for dividend reinvestment plan	374	313
Stock-based compensation capitalized	701	696
Contributions from limited partners in consolidated partnerships, net	13	58
Non-controlling interest recorded, fair value	0	15,385
Common stock issued for dividend reinvestment in trust	214	189
Contribution of stock awards into trust	1,042	1,440
Distribution of stock held in trust	$ 0	$ 4