Consolidated Statements of Changes in Equity (USD $)	Total	Preferred Stock	Common Stock	Paid - in Capital	Distributions in Excess of Net Income	Accumulated Other Comprehensive Income/(Loss)	Noncontrolling Interest
Beginning Balance at Dec. 31, 2011	$ 2,318,784,000	$ 128,180,000	$ 2,197,000	$ 3,340,470,000	$ (1,142,895,000)	$ (13,902,000)	$ 4,734,000
Beginning Balance, Shares at Dec. 31, 2011		6,068,174	219,650,225
Consolidated Statements of Changes in Equity
Net income	212,177,000				212,177,000
Net income/(loss) attributable to non-controlling interests	140,000						140,000
Other comprehensive (loss)/income	2,645,000					2,645,000
Issuance/(forfeiture) of common and restricted shares, net, Shares			(22,224)
Issuance/(forfeiture) of common and restricted shares, net	(742,000)		0	(742,000)
Issuance of common shares through public offering, Shares			30,490,969
Issuance of common shares through public offering	756,138,000		305,000	755,833,000
Redemption of 6.75% Series G Cumulative Redeemable Shares, Shares		(3,264,362)
Redemption of 6.75% Series G Cumulative Redeemable Shares	(81,609,000)	(81,609,000)		2,791,000	(2,791,000)
Adjustment for conversion of non-controlling interest of unitholders in Operating Partnership, Shares			20,438
Adjustment for conversion of non-controlling interest of unitholders in Operating Partnership	529,000		(1,000)	530,000
Acquisition of non-controlling interest	(4,871,000)		0	0	0	0	(4,871,000)
Increase in non-controlling interest from business combination, net	913,000						913,000
Common stock distributions declared	(215,654,000)				(215,654,000)	0
Preferred stock distributions declared-Series E	(3,724,000)				(3,724,000)	0
Preferred stock distributions declared-Series G	(2,286,000)				(2,286,000)	0
Adjustment to reflect redemption value of redeemable non-controlling interests	11,392,000				11,392,000	0
Ending Balance at Dec. 31, 2012	2,993,832,000	46,571,000	2,501,000	4,098,882,000	(1,143,781,000)	(11,257,000)	916,000
Ending Balance, Shares at Dec. 31, 2012		2,803,812	250,139,408
Consolidated Statements of Changes in Equity
Net income	44,812,000				44,812,000
Net income/(loss) attributable to non-controlling interests	(60,000)						(60,000)
Other comprehensive (loss)/income	6,132,000					6,132,000
Issuance/(forfeiture) of common and restricted shares, net, Shares			533,966
Issuance/(forfeiture) of common and restricted shares, net	9,072,000		5,000	9,067,000
Issuance of common shares through public offering	0
Redemption of 6.75% Series G Cumulative Redeemable Shares, Shares
Adjustment for conversion of non-controlling interest of unitholders in Operating Partnership, Shares			76,291
Adjustment for conversion of non-controlling interest of unitholders in Operating Partnership	1,817,000		1,000	1,816,000
Acquisition of non-controlling interest							(4,900,000)
Common stock distributions declared	(235,721,000)				(235,721,000)	0
Preferred stock distributions declared-Series E	(3,724,000)				(3,724,000)	0
Adjustment to reflect redemption value of redeemable non-controlling interests	(3,656,000)				(3,656,000)	0
Ending Balance at Dec. 31, 2013	2,812,504,000	46,571,000	2,507,000	4,109,765,000	(1,342,070,000)	(5,125,000)	856,000
Ending Balance, Shares at Dec. 31, 2013		2,803,812	250,749,665
Consolidated Statements of Changes in Equity
Net income	154,334,000				154,334,000
Net income/(loss) attributable to non-controlling interests	(3,000)						(3,000)
Other comprehensive (loss)/income	(3,730,000)					(3,730,000)
Issuance/(forfeiture) of common and restricted shares, net, Shares			801,054
Issuance/(forfeiture) of common and restricted shares, net	9,805,000		8,000	9,797,000
Issuance of common shares through public offering, Shares			3,410,433
Issuance of common shares through public offering	99,849,000		34,000	99,815,000
Redemption of 6.75% Series G Cumulative Redeemable Shares, Shares		0
Adjustment for conversion of non-controlling interest of unitholders in Operating Partnership, Shares	153,451		153,451
Adjustment for conversion of non-controlling interest of unitholders in Operating Partnership	4,372,000		2,000	4,370,000
Common stock distributions declared	(263,503,000)				(263,503,000)
Preferred stock distributions declared-Series E	(3,724,000)				(3,724,000)
Adjustment to reflect redemption value of redeemable non-controlling interests	(73,954,000)				(73,954,000)
Ending Balance at Dec. 31, 2014	$ 2,735,950,000	$ 46,571,000	$ 2,551,000	$ 4,223,747,000	$ (1,528,917,000)	$ (8,855,000)	$ 853,000
Ending Balance, Shares at Dec. 31, 2014		2,803,812	255,114,603