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, 2011	$ 5,413,516	$ 1,481		$ 4,936,457	$ (53,080)	$ (2,722)	$ (16,138)	$ 547,518
Equity, shares at Dec. 31, 2011		148,108,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of operating partnership units to Common Stock, shares		227,000
Conversion of operating partnership units to Common Stock, value	0	2		6,829				(6,831)
Conversion of redeemable preferred units to common units	4,115							4,115
Allocated net income for the year	188,443				166,971			21,472
Dividends/distributions declared					(165,055)
Sale of common stock, net of offering costs		2,348,000
Sale of common stock, net of offering costs	247,186	24		247,162
Shares issued in connection with exchange of Exchangeable Senior Notes, value	0
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		29,000
Net activity from stock option and incentive plan, value	18,705			4,032				14,673
Noncontrolling interests in property partnerships recorded upon consolidation	0
Distributions to noncontrolling interests in property partnerships	(1,500)							(1,500)
Amortization of interest rate contracts	1,297						1,160	137
Reallocation of noncontrolling interest				(10,178)				10,178
Equity, value at Jun. 30, 2012	5,686,988	1,507		5,184,683	(51,164)	(2,722)	(14,978)	569,662
Equity, shares at Jun. 30, 2012		150,716,000
Equity, value at Dec. 31, 2012	5,634,854	1,516	0	5,222,073	(109,985)	(2,722)	(13,817)	537,789
Equity, shares at Dec. 31, 2012	151,601,209	151,601,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of operating partnership units to Common Stock, shares		334,000
Conversion of operating partnership units to Common Stock, value	0	4		10,305				(10,309)
Allocated net income for the year	558,835				502,857			55,978
Dividends/distributions declared	(223,085)				(200,380)			(22,705)
Issuance of 5.25% Series B cumulative redeemable preferred stock	193,920		200,000
Shares issued in connection with exchange of Exchangeable Senior Notes, shares		419,000
Shares issued in connection with exchange of Exchangeable Senior Notes, value	43,834	4		43,830
Equity component of exchange of Exchangeable Senior Notes				(43,869)
Issuance of 5.25% Series B cumulative redeemable preferred stock				(6,080)
Shares issued pursuant to stock purchase plan, shares		3,000
Shares issued pursuant to stock purchase plan, value	335			335
Net activity from stock option and incentive plan, shares		28,000
Net activity from stock option and incentive plan, value	25,741			5,353				20,388
Noncontrolling interests in property partnerships recorded upon consolidation	483,488							483,488
Contributions from noncontrolling interests in property partnerships	10,564							10,564
Distributions to noncontrolling interests in property partnerships	(1,500)							(1,500)
Amortization of interest rate contracts	1,255						1,128	127
Reallocation of noncontrolling interest	0			14,296				(14,296)
Equity, value at Jun. 30, 2013	$ 6,684,372	$ 1,524	$ 200,000	$ 5,246,243	$ 192,492	$ (2,722)	$ (12,689)	$ 1,059,524
Equity, shares at Jun. 30, 2013	152,384,740	152,385,000