Consolidated Statements Of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Preferred 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, 2013	$ 7,043,618	$ 1,530	$ 200,000	$ 5,662,453	$ (108,552)	$ (2,722)	$ (11,556)	$ 1,302,465
Equity, shares at Dec. 31, 2013		152,983,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of operating partnership units to Common Stock, shares		18,000
Conversion of operating partnership units to Common Stock, value	0	0		627				(627)
Allocated net income for the year	65,369				56,623			8,746
Dividends/distributions declared	(113,268)				(102,050)
Dividends/distributions declared								(11,218)
Shares issued pursuant to stock purchase plan, shares		4,000
Shares issued pursuant to stock purchase plan, value	357			357
Net activity from stock option and incentive plan, shares		12,000
Net activity from stock option and incentive plan, value	9,464	0		704				8,760
Contributions from noncontrolling interests in property partnerships	468							468
Distributions to noncontrolling interests in property partnerships	(4,093)							(4,093)
Amortization of interest rate contracts	629						567	62
Reallocation of noncontrolling interest	0			4,089				(4,089)
Equity, value at Mar. 31, 2014	7,002,544	1,530	200,000	5,668,230	(153,979)	(2,722)	(10,989)	1,300,474
Equity, shares at Mar. 31, 2014		153,017,000
Equity, value at Dec. 31, 2014	7,902,936	1,531	200,000	6,270,257	(762,464)	(2,722)	(9,304)	2,205,638
Equity, shares at Dec. 31, 2014	153,113,945	153,114,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of operating partnership units to Common Stock, shares		259,000
Conversion of operating partnership units to Common Stock, value		3		8,686				(8,689)
Allocated net income for the year	206,939				173,771			33,168
Dividends/distributions declared	(114,005)				(102,300)
Dividends/distributions declared								(11,705)
Shares issued pursuant to stock purchase plan, shares		2,000
Shares issued pursuant to stock purchase plan, value	313			313
Net activity from stock option and incentive plan, shares		27,000
Net activity from stock option and incentive plan, value	21,616			1,842				19,774
Contributions from noncontrolling interests in property partnerships	629							629
Distributions to noncontrolling interests in property partnerships	(16,574)							(16,574)
Effective portion of interest rate contracts	(3,533)						(3,165)	(368)
Amortization of interest rate contracts	627						562	65
Reallocation of noncontrolling interest				5,162				(5,162)
Equity, value at Mar. 31, 2015	$ 7,998,948	$ 1,534	$ 200,000	$ 6,286,260	$ (690,993)	$ (2,722)	$ (11,907)	$ 2,216,776
Equity, shares at Mar. 31, 2015	153,402,107	153,402,000