Consolidated Statements of Cash Flows (USD $)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Document Fiscal Year Focus	2014
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation, net of capitalization	$ 4,534,000	$ 6,159,000
Cash flows from financing activities:
Net proceeds from common stock issuance		99,774,000
Parent Company [Member]
Proceeds from Issuance of Unsecured Debt	248,705,000	0
Cash flows from operating activities:
Net income	56,217,000	58,603,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	73,928,000	65,170,000
Amortization of deferred loan cost and debt premium	5,675,000	6,175,000
Accretion of above and below market lease intangibles, net	(1,698,000)	(1,042,000)
Stock-based compensation, net of capitalization	4,534,000	6,159,000
Equity in income of investments in real estate partnerships	(16,640,000)	(11,888,000)
Net gain on sale of properties	(2,406,000)	(13,127,000)
Provision for impairment	(225,000)	0
Distribution of earnings from operations of investments in real estate partnerships	(18,736,000)	(24,376,000)
Loss on settlement of derivative instruments	(4,648,000)	0
Loss on derivative instruments	(9,000)	(9,000)
Deferred compensation expense	830,000	1,051,000
Realized and unrealized gains on trading securities held in trust	(847,000)	(1,051,000)
Changes in assets and liabilities:
Restricted cash	37,000	1,118,000
Accounts receivable	(10,365,000)	(328,000)
Straight-line rent receivables, net	(3,062,000)	(2,612,000)
Deferred leasing costs	(5,323,000)	(4,212,000)
Other assets	(2,016,000)	(3,175,000)
Accounts payable and other liabilities	(1,964,000)	(12,028,000)
Tenants’ security and escrow deposits and prepaid rent	(904,000)	(3,846,000)
Net cash provided by operating activities	119,596,000	109,334,000
Cash flows from investing activities:
Acquisition of operating real estate	(79,444,000)	(26,676,000)
Development of real estate, including acquisition of land	(93,764,000)	(84,209,000)
Proceeds from sale of real estate investments	7,790,000	84,699,000
Collection of notes receivable	0	6,025,000
Investments in real estate partnerships	(4,287,000)	(8,060,000)
Distributions received from investments in real estate partnerships	21,496,000	11,457,000
Dividends on trading securities held in trust	66,000	70,000
Acquisition of securities	(18,195,000)	(15,679,000)
Proceeds from sale of securities	3,702,000	10,632,000
Net cash used in investing activities	(162,636,000)	(21,741,000)
Cash flows from financing activities:
Net proceeds from common stock issuance	0	98,278,000
Proceeds from sale of treasury stock	0	34,000
Redemption of preferred stock and partnership units	(300,000)	0
Distributions to limited partners in consolidated partnerships, net	(938,000)	(3,272,000)
Distributions to exchangeable operating partnership unit holders	(152,000)	(171,000)
Dividends paid to common stockholders	(85,928,000)	(83,296,000)
Dividends paid to preferred stockholders	(10,531,000)	(5,265,000)
Proceeds from unsecured credit facilities	245,000,000	77,000,000
Repayment of unsecured credit facilities	(235,000,000)	(122,000,000)
Proceeds from notes payable	655,000	8,250,000
Repayments of Notes Payable	6,615,000	16,349,000
Scheduled principal payments	(3,413,000)	(3,893,000)
Payment of loan costs	(2,391,000)	(115,000)
Net cash used in financing activities	(908,000)	(50,799,000)
Repayments of Unsecured Debt	150,000,000	0
Net (decrease) increase in cash and cash equivalents	(43,948,000)	36,794,000
Cash and cash equivalents at beginning of the period	80,684,000	22,349,000
Cash and cash equivalents at end of the period	36,736,000	59,143,000
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $3,272 and $2,305 in 2014 and 2013, respectively)	54,083,000	54,670,000
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	0	5,266,000
Real estate received through distribution in kind	0	7,576,000
Mortgage loans assumed through distribution in kind	0	7,500,000
Mortgage loans assumed for the acquisition of real estate, net of premiums	78,049,000	0
Change in fair value of derivative instruments	24,646,000	21,720,000
Common stock issued for dividend reinvestment plan	604,000	578,000
Stock-based compensation capitalized	1,410,000	948,000
Contributions from limited partners in consolidated partnerships, net	95,000	0
Common stock issued for dividend reinvestment in trust	384,000	320,000
Contribution of stock awards into trust	1,846,000	1,504,000
Distribution of stock held in trust	4,000	201,000
Available-for-sale Securities, Gross Unrealized Gain (Loss)	914,000	0
Partnership Interest [Member]
Proceeds from Issuance of Unsecured Debt	248,705,000	0
Cash flows from operating activities:
Net income	56,217,000	58,603,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	73,928,000	65,170,000
Amortization of deferred loan cost and debt premium	5,675,000	6,175,000
Accretion of above and below market lease intangibles, net	(1,698,000)	(1,042,000)
Stock-based compensation, net of capitalization	4,534,000	6,159,000
Equity in income of investments in real estate partnerships	(16,640,000)	(11,888,000)
Net gain on sale of properties	(2,406,000)	(13,127,000)
Provision for impairment	(225,000)	0
Distribution of earnings from operations of investments in real estate partnerships	(18,736,000)	(24,376,000)
Loss on settlement of derivative instruments	(4,648,000)	0
Loss on derivative instruments	(9,000)	(9,000)
Deferred compensation expense	830,000	1,051,000
Realized and unrealized gains on trading securities held in trust	(847,000)	(1,051,000)
Changes in assets and liabilities:
Restricted cash	37,000	1,118,000
Accounts receivable	(10,365,000)	(328,000)
Straight-line rent receivables, net	(3,062,000)	(2,612,000)
Deferred leasing costs	(5,323,000)	(4,212,000)
Other assets	(2,016,000)	(3,175,000)
Accounts payable and other liabilities	(1,964,000)	(12,028,000)
Tenants’ security and escrow deposits and prepaid rent	(904,000)	(3,846,000)
Net cash provided by operating activities	119,596,000	109,334,000
Cash flows from investing activities:
Acquisition of operating real estate	(79,444,000)	(26,676,000)
Development of real estate, including acquisition of land	(93,764,000)	(84,209,000)
Proceeds from sale of real estate investments	7,790,000	84,699,000
Collection of notes receivable	0	6,025,000
Investments in real estate partnerships	(4,287,000)	(8,060,000)
Distributions received from investments in real estate partnerships	21,496,000	11,457,000
Dividends on trading securities held in trust	66,000	70,000
Acquisition of securities	(18,195,000)	(15,679,000)
Proceeds from sale of securities	3,702,000	10,632,000
Net cash used in investing activities	(162,636,000)	(21,741,000)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	0	98,278,000
Proceeds from sale of treasury stock	0	34,000
Redemption of preferred stock and partnership units	(300,000)	0
Distributions to limited partners in consolidated partnerships, net	(938,000)	(3,272,000)
Dividends paid to common stockholders	(86,080,000)	(83,467,000)
Dividends paid to preferred stockholders	(10,531,000)	(5,265,000)
Proceeds from unsecured credit facilities	245,000,000	77,000,000
Repayment of unsecured credit facilities	(235,000,000)	(122,000,000)
Proceeds from notes payable	655,000	8,250,000
Repayments of Notes Payable	6,615,000	16,349,000
Scheduled principal payments	(3,413,000)	(3,893,000)
Payment of loan costs	(2,391,000)	(115,000)
Net cash used in financing activities	(908,000)	(50,799,000)
Repayments of Unsecured Debt	150,000,000	0
Net (decrease) increase in cash and cash equivalents	(43,948,000)	36,794,000
Cash and cash equivalents at beginning of the period	80,684,000	22,349,000
Cash and cash equivalents at end of the period	36,736,000	59,143,000
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $3,272 and $2,305 in 2014 and 2013, respectively)	54,083,000	54,670,000
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	0	5,266,000
Real estate received through distribution in kind	0	7,576,000
Mortgage loans assumed through distribution in kind	0	7,500,000
Mortgage loans assumed for the acquisition of real estate, net of premiums	78,049,000	0
Change in fair value of derivative instruments	24,646,000	21,720,000
Common stock issued for dividend reinvestment plan	604,000	578,000
Stock-based compensation capitalized	1,410,000	948,000
Contributions from limited partners in consolidated partnerships, net	95,000	0
Non-controlling interest recorded, fair value		0
Common stock issued for dividend reinvestment in trust	384,000	320,000
Contribution of stock awards into trust	1,846,000	1,504,000
Distribution of stock held in trust	4,000	201,000
Available-for-sale Securities, Gross Unrealized Gain (Loss)	$ 914,000	$ 0