Equity - Reconciliation of total stockholders' equity (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			$ 1,245,286
Issuance of common stock			27,887
Common stock redemptions			(382)
Stock-based compensation			2,782
Net income	6,011	(24,238)	9,974	(25,256)
Dividends to common stockholders ($0.60 per share)			(57,660)
Distributions to non-controlling interests			(348)
Purchase of noncontrolling interest in consolidated partnership			(8,189)
Balance at June 30, 2014	1,219,350		1,219,350
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			959
Issuance of common stock			12
Common stock redemptions
Stock-based compensation			1
Net income
Dividends to common stockholders ($0.60 per share)
Distributions to non-controlling interests
Purchase of noncontrolling interest in consolidated partnership
Balance at June 30, 2014	972		972
Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			2,325,228
Issuance of common stock			27,875
Common stock redemptions			(382)
Stock-based compensation			2,781
Net income
Dividends to common stockholders ($0.60 per share)
Distributions to non-controlling interests
Purchase of noncontrolling interest in consolidated partnership			(6,577)
Adjustments to Additional Paid in Capital, Redemptions or Purchase of Interests			6,577
Balance at June 30, 2014	2,348,925		2,348,925
Accumulated Other Comprehensive Loss [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			51
Issuance of common stock
Common stock redemptions
Stock-based compensation
Net income
Dividends to common stockholders ($0.60 per share)
Distributions to non-controlling interests
Purchase of noncontrolling interest in consolidated partnership
Balance at June 30, 2014	51		51
Cumulative Net Income Attributable to Common Stockholders [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			808,362
Issuance of common stock
Common stock redemptions
Stock-based compensation
Net income			9,823
Dividends to common stockholders ($0.60 per share)
Distributions to non-controlling interests
Purchase of noncontrolling interest in consolidated partnership
Balance at June 30, 2014	818,185		818,185
Cumulative Dividends [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			(1,891,123)
Issuance of common stock
Common stock redemptions
Stock-based compensation
Net income
Dividends to common stockholders ($0.60 per share)			(57,660)
Distributions to non-controlling interests
Purchase of noncontrolling interest in consolidated partnership
Balance at June 30, 2014	(1,948,783)		(1,948,783)
Total Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			1,243,477
Issuance of common stock			27,887
Common stock redemptions			(382)
Stock-based compensation			2,782
Net income			9,823
Dividends to common stockholders ($0.60 per share)			(57,660)
Distributions to non-controlling interests
Balance at June 30, 2014	1,219,350		1,219,350
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2013			1,809
Issuance of common stock
Common stock redemptions
Stock-based compensation
Net income			151
Dividends to common stockholders ($0.60 per share)
Distributions to non-controlling interests			(348)
Purchase of noncontrolling interest in consolidated partnership			(1,612)
Balance at June 30, 2014