Consolidated Statements Of Stockholders' Equity (USD $) In Thousands, except Share data	Common Stock [Member]	Additional Paid-In Capital [Member]	Dividends In Excess Of Earnings [Member]	Treasury Stock, At Cost [Member]	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests [Member]	Total
Equity, value at Dec. 31, 2010	$ 1,402	$ 4,417,162	$ (24,763)	$ (2,722)	$ (18,436)	$ 591,550	$ 4,964,193
Equity, shares at Dec. 31, 2010	140,199,000
Conversion of operating partnership units to Common Stock, shares	1,031,000
Conversion of operating partnership units to Common Stock, value	11	29,205				(29,216)
Reallocation of noncontrolling interest		(25,778)				25,778
Allocated net income for the year			40,813			6,558	47,371
Dividends/distributions declared			(72,529)			(10,078)	(82,607)
Sale of common stock, net of offering costs, shares	3,695,000
Sale of common stock, net of offering costs, value	37	345,332					345,369
Shares issued pursuant to stock purchase plan, shares	4,000
Shares issued pursuant to stock purchase plan, value		312					312
Net activity from stock option and incentive plan, shares	129,000
Net activity from stock option and incentive plan, value	1	5,426				9,245	14,672
Distributions to noncontrolling interests in property partnerships						(500)	(500)
Amortization of interest rate contracts					569	80	649
Equity, value at Mar. 31, 2011	1,451	4,771,659	(56,479)	(2,722)	(17,867)	593,417	5,289,459
Equity, shares at Mar. 31, 2011	145,058,000
Equity, value at Dec. 31, 2011	1,481	4,936,457	(53,080)	(2,722)	(16,138)	547,518	5,413,516
Equity, shares at Dec. 31, 2011	148,108,000						148,107,611
Conversion of operating partnership units to Common Stock, shares	200,000
Conversion of operating partnership units to Common Stock, value	2	6,024				(6,026)
Reallocation of noncontrolling interest		(3,638)				3,638
Conversion of redeemable preferred units to common units						4,115	4,115
Allocated net income for the year			64,632			8,577	73,209
Dividends/distributions declared			(82,161)			(10,064)	(92,225)
Sale of common stock, net of offering costs, shares	1,049,000
Sale of common stock, net of offering costs, value	11	109,319					109,330
Shares issued pursuant to stock purchase plan, shares	4,000
Shares issued pursuant to stock purchase plan, value		381					381
Net activity from stock option and incentive plan, shares	23,000
Net activity from stock option and incentive plan, value		2,004				9,586	11,590
Distributions to noncontrolling interests in property partnerships						(750)	(750)
Amortization of interest rate contracts					580	69	649
Equity, value at Mar. 31, 2012	$ 1,494	$ 5,050,547	$ (70,609)	$ (2,722)	$ (15,558)	$ 556,663	$ 5,519,815
Equity, shares at Mar. 31, 2012	149,384,000						149,384,341