Consolidated Statements Of Stockholders' Equity (USD $) In Thousands, except Share data	Total	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]
Equity, value at Dec. 31, 2010	$ 4,964,193	$ 1,402	$ 4,417,162	$ (24,763)	$ (2,722)	$ (18,436)	$ 591,550
Equity, shares at Dec. 31, 2010		140,199,000
Conversion of operating partnership units to common stock, shares		2,443,000
Conversion of operating partnership units to common stock, value		24	71,157				(71,181)
Allocated net income for the year	195,886			171,359			24,527
Dividends/distributions declared	(248,657)			(219,537)			(29,120)
Sale of common stock, net of offering costs, shares		4,660,000
Sale of common stock, net of offering costs, value	439,037	47	438,990
Shares issued pursuant to stock purchase plan, shares		7,000
Shares issued pursuant to stock purchase plan, value	620		620
Net activity from stock option and incentive plan, shares		318,000
Net activity from stock option and incentive plan, value	32,636	3	15,266				17,367
Distributions to noncontrolling interests in property partnerships	(1,500)						(1,500)
Amortization of interest rate contracts	1,946					1,719	227
Reallocation of noncontrolling interest			(26,755)				26,755
Equity, value at Sep. 30, 2011	5,384,161	1,476	4,916,440	(72,941)	(2,722)	(16,717)	558,625
Equity, shares at Sep. 30, 2011		147,627,000
Equity, value at Dec. 31, 2011	5,413,516	1,481	4,936,457	(53,080)	(2,722)	(16,138)	547,518
Equity, shares at Dec. 31, 2011	148,107,611	148,108,000
Conversion of operating partnership units to common stock, shares		366,000
Conversion of operating partnership units to common stock, value		4	11,232				(11,236)
Conversion of redeemable preferred units to common units	5,852						5,852
Allocated net income for the year	272,419			241,473			30,946
Dividends/distributions declared	(278,114)			(248,014)			(30,100)
Sale of common stock, net of offering costs, shares		2,348,000
Sale of common stock, net of offering costs, value	247,186	24	247,162
Shares issued pursuant to stock purchase plan, shares		8,000
Shares issued pursuant to stock purchase plan, value	781		781
Net activity from stock option and incentive plan, shares		26,000
Net activity from stock option and incentive plan, value	24,294		4,473				19,821
Distributions to noncontrolling interests in property partnerships	(2,250)						(2,250)
Amortization of interest rate contracts	1,967					1,759	208
Reallocation of noncontrolling interest			(5,536)				5,536
Equity, value at Sep. 30, 2012	$ 5,685,651	$ 1,509	$ 5,194,569	$ (59,621)	$ (2,722)	$ (14,379)	$ 566,295
Equity, shares at Sep. 30, 2012	150,856,237	150,856,000