Consolidated Statements of Cash Flows (USD $) Share data in Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 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	$ 6,885,000	$ 9,227,000
Cash flows from financing activities:
Net proceeds from common stock issuance	49,995,000	101,342,000
Parent Company [Member]
Incremental Common Shares Attributable to Dilutive Effect of Equity Forward Agreements	0	0
Noncash or Part Noncash Acquisition, Other Assets Acquired	0	7,646,000
Proceeds from Issuance of Unsecured Debt	248,705,000	0
Cash flows from operating activities:
Net income	109,659,000	99,268,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	110,355,000	98,620,000
Amortization of deferred loan cost and debt premium	8,095,000	9,265,000
Accretion of above and below market lease intangibles, net	(2,315,000)	(1,646,000)
Stock-based compensation, net of capitalization	6,885,000	9,227,000
Equity in income of investments in real estate partnerships	(22,353,000)	(25,150,000)
Net gain on sale of properties	(29,598,000)	(29,235,000)
Provision for impairment	(225,000)	(6,000,000)
Distribution of earnings from operations of investments in real estate partnerships	(30,008,000)	(34,772,000)
Loss on settlement of derivative instruments	(4,648,000)	0
Hedge ineffectiveness	(13,000)	(14,000)
Deferred compensation expense	610,000	2,023,000
Realized and unrealized gains on trading securities held in trust	(612,000)	(2,024,000)
Changes in assets and liabilities:
Restricted cash	497,000	1,185,000
Accounts receivable	(2,801,000)	(2,200,000)
Straight-line rent receivables, net	(4,724,000)	(3,850,000)
Deferred leasing costs	(6,416,000)	(6,599,000)
Other assets	131,000	(1,767,000)
Accounts payable and other liabilities	15,018,000	8,137,000
Tenants’ security and escrow deposits and prepaid rent	511,000	4,550,000
Net cash provided by operating activities	217,810,000	200,562,000
Cash flows from investing activities:
Acquisition of operating real estate	(98,018,000)	(26,676,000)
Development of real estate, including acquisition of land	(160,552,000)	(162,419,000)
Proceeds from sale of real estate investments	62,788,000	136,997,000
Collection of notes receivable	0	6,015,000
Investments in real estate partnerships	(6,012,000)	(10,844,000)
Distributions received from investments in real estate partnerships	29,916,000	31,457,000
Dividends on trading securities held in trust	100,000	95,000
Acquisition of securities	(19,866,000)	(17,795,000)
Proceeds from sale of securities	5,344,000	12,732,000
Net cash used in investing activities	(186,300,000)	(30,438,000)
Cash flows from financing activities:
Net proceeds from common stock issuance	49,300,000	99,753,000
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	(4,619,000)	(3,288,000)
Distributions to exchangeable operating partnership unit holders	(228,000)	(245,000)
Dividends paid to common stockholders	(129,044,000)	(125,528,000)
Dividends paid to preferred stockholders	(15,797,000)	(15,797,000)
Proceeds from unsecured credit facilities	255,000,000	82,000,000
Repayment of unsecured credit facilities	(255,000,000)	(152,000,000)
Proceeds from notes payable	12,025,000	8,250,000
Repayments of Notes Payable	13,487,000	16,439,000
Scheduled principal payments	(5,068,000)	(6,352,000)
Payment of loan costs	(2,973,000)	(159,000)
Net cash used in financing activities	(11,486,000)	(129,771,000)
Repayments of Unsecured Debt	150,000,000	0
Net increase in cash and cash equivalents	20,024,000	40,353,000
Cash and cash equivalents at beginning of the period	80,684,000	22,349,000
Cash and cash equivalents at end of the period	100,708,000	62,702,000
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $ 5,158 and $4,174 in 2014 and 2013, respectively)	72,573,000	72,607,000
Supplemental disclosure of non-cash transactions:
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	(28,144,000)	22,249,000
Common stock issued for dividend reinvestment plan	895,000	831,000
Stock-based compensation capitalized	2,026,000	1,567,000
Contributions from limited partners in consolidated partnerships, net	116,000	0
Common stock issued for dividend reinvestment in trust	581,000	489,000
Contribution of stock awards into trust	1,865,000	1,522,000
Distribution of stock held in trust	4,000	201,000
Available-for-sale Securities, Gross Unrealized Gain (Loss)	4,809,000	0
Partnership Interest [Member]
Noncash or Part Noncash Acquisition, Other Assets Acquired	0	7,646,000
Proceeds from Issuance of Unsecured Debt	248,705,000	0
Cash flows from operating activities:
Net income	109,659,000	99,268,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	110,355,000	98,620,000
Amortization of deferred loan cost and debt premium	8,095,000	9,265,000
Accretion of above and below market lease intangibles, net	(2,315,000)	(1,646,000)
Stock-based compensation, net of capitalization	6,885,000	9,227,000
Equity in income of investments in real estate partnerships	(22,353,000)	(25,150,000)
Net gain on sale of properties	(29,598,000)	(29,235,000)
Provision for impairment	(225,000)	(6,000,000)
Distribution of earnings from operations of investments in real estate partnerships	(30,008,000)	(34,772,000)
Loss on settlement of derivative instruments	(4,648,000)	0
Hedge ineffectiveness	(13,000)	(14,000)
Deferred compensation expense	610,000	2,023,000
Realized and unrealized gains on trading securities held in trust	(612,000)	(2,024,000)
Changes in assets and liabilities:
Restricted cash	497,000	1,185,000
Accounts receivable	(2,801,000)	(2,200,000)
Straight-line rent receivables, net	(4,724,000)	(3,850,000)
Deferred leasing costs	(6,416,000)	(6,599,000)
Other assets	131,000	(1,767,000)
Accounts payable and other liabilities	15,018,000	8,137,000
Tenants’ security and escrow deposits and prepaid rent	511,000	4,550,000
Net cash provided by operating activities	217,810,000	200,562,000
Cash flows from investing activities:
Acquisition of operating real estate	(98,018,000)	(26,676,000)
Development of real estate, including acquisition of land	(160,552,000)	(162,419,000)
Proceeds from sale of real estate investments	62,788,000	136,997,000
Collection of notes receivable	0	6,015,000
Investments in real estate partnerships	(6,012,000)	(10,844,000)
Distributions received from investments in real estate partnerships	29,916,000	31,457,000
Dividends on trading securities held in trust	100,000	95,000
Acquisition of securities	(19,866,000)	(17,795,000)
Proceeds from sale of securities	5,344,000	12,732,000
Net cash used in investing activities	(186,300,000)	(30,438,000)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	49,300,000	99,753,000
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	(4,619,000)	(3,288,000)
Dividends paid to common stockholders	(129,272,000)	(125,773,000)
Dividends paid to preferred stockholders	(15,797,000)	(15,797,000)
Proceeds from unsecured credit facilities	255,000,000	82,000,000
Repayment of unsecured credit facilities	(255,000,000)	(152,000,000)
Proceeds from notes payable	12,025,000	8,250,000
Repayments of Notes Payable	13,487,000	16,439,000
Scheduled principal payments	(5,068,000)	(6,352,000)
Payment of loan costs	(2,973,000)	(159,000)
Net cash used in financing activities	(11,486,000)	(129,771,000)
Repayments of Unsecured Debt	150,000,000	0
Net increase in cash and cash equivalents	20,024,000	40,353,000
Cash and cash equivalents at beginning of the period	80,684,000	22,349,000
Cash and cash equivalents at end of the period	100,708,000	62,702,000
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $ 5,158 and $4,174 in 2014 and 2013, respectively)	72,573,000	72,607,000
Supplemental disclosure of non-cash transactions:
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	(28,144,000)	22,249,000
Common stock issued for dividend reinvestment plan	895,000	831,000
Stock-based compensation capitalized	2,026,000	1,567,000
Contributions from limited partners in consolidated partnerships, net	116,000	0
Non-controlling interest recorded, fair value		0
Common stock issued for dividend reinvestment in trust	581,000	489,000
Contribution of stock awards into trust	1,865,000	1,522,000
Distribution of stock held in trust	4,000	201,000
Available-for-sale Securities, Gross Unrealized Gain (Loss)	$ 4,809,000	$ 0