CONSOLIDATED STATEMENTS OF EQUITY (USD $)	Total	Common Stock	Additional Paid-In Capital	Distributions in Excess of Accumulated Earnings	Accumulated Other Comprehensive Income (Loss)	Non-controlling Interest
Balance at Dec. 31, 2009	$ 888,074,000	$ 347,000	$ 922,078,000	$ (261,313,000)	$ 0	$ 226,962,000
Balance (in shares) at Dec. 31, 2009		34,655,428
Increase (Decrease) in Stockholders' Equity
Net income	26,318,000			20,081,000		6,237,000
Issuance of common stock, net	123,756,000	28,000	123,728,000
Issuance of common stock, net (in shares)		2,827,856
Stock-based compensation	7,647,000		7,647,000
Stock-based compensation (in shares)		6,206
Repurchase of common stock	(3,274,000)	(1,000)	(3,273,000)
Repurchase of common stock (in shares)		(68,265)
Conversion of UPREIT Units for common stock	0	5,000	10,229,000			(10,234,000)
Conversion of UPREIT Units for common stock (in shares)		528,004
Issuance of UPREIT Units associated with property acquisition	4,845,000					4,845,000
Adjustment of noncontrolling interest	0		(13,084,000)			13,084,000
Dividends and distributions paid	(112,232,000)			(85,579,000)		(26,653,000)
Balance at Dec. 31, 2010	935,134,000	379,000	1,047,325,000	(326,811,000)	0	214,241,000
Balance (in shares) at Dec. 31, 2010		37,949,229
Increase (Decrease) in Stockholders' Equity
Net income	47,664,000			37,856,000		9,808,000
Issuance of common stock, net	216,603,000	39,000	216,564,000
Issuance of common stock, net (in shares)		3,881,833
Issuance of common stock through public offering, net	336,760,000	60,000	336,700,000
Issuance of common stock through public offering, net (in shares)		6,000,000
Stock-based compensation	10,105,000		10,105,000
Stock-based compensation (in shares)		21,457
Repurchase of common stock	(5,770,000)	(1,000)	(5,769,000)
Repurchase of common stock (in shares)		(96,723)
Conversion of UPREIT Units for common stock	0	6,000	11,393,000			(11,399,000)
Conversion of UPREIT Units for common stock (in shares)		565,509
Adjustment of noncontrolling interest	0		(70,755,000)			70,755,000
Dividends and distributions paid	(130,852,000)			(103,423,000)		(27,429,000)
Balance at Dec. 31, 2011	1,409,644,000	483,000	1,545,563,000	(392,378,000)	0	255,976,000
Balance (in shares) at Dec. 31, 2011	48,321,305	48,321,305
Increase (Decrease) in Stockholders' Equity
Net income	163,622,000			135,302,000		28,320,000
Unrealized gain (loss) on interest rate swap agreements	(1,296,000)				(1,069,000)	(227,000)
Issuance of common stock, net	165,968,000	30,000	165,938,000
Issuance of common stock, net (in shares)		2,966,742
Stock-based compensation	13,923,000		13,923,000
Stock-based compensation (in shares)		4,199
Repurchase of common stock	(4,245,000)	(1,000)	(4,244,000)
Repurchase of common stock (in shares)		(68,284)
Conversion of UPREIT Units for common stock	0	3,000	6,896,000			(6,899,000)
Conversion of UPREIT Units for common stock (in shares)		284,180
Adjustment of noncontrolling interest	0		(18,157,000)			18,157,000
Dividends and distributions paid	(159,349,000)			(131,321,000)		(28,028,000)
Balance at Dec. 31, 2012	$ 1,588,267,000	$ 515,000	$ 1,709,919,000	$ (388,397,000)	$ (1,069,000)	$ 267,299,000
Balance (in shares) at Dec. 31, 2012	51,508,142	51,508,142