Stockholders' Equity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Dividends per share declared to common stockholders (in dollars per share)	$ 0.30	$ 0.30	$ 0.60	$ 0.60
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			$ 1,120,944
Issuance of common stock			132,417
Common stock redemptions			(246)
Stock-based compensation			2,985
Total comprehensive loss	(24,238)	2,928	(25,256)	6,062
Dividends to common stockholders ($0.60 per share)			(54,061)
Distributions to non-controlling interests			(3)
Proceeds from non-controlling interests			1,388
Ending Balance	1,178,168		1,178,168
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			875
Issuance of common stock			49
Common stock redemptions			0
Stock-based compensation			1
Total comprehensive loss			0
Dividends to common stockholders ($0.60 per share)			0
Distributions to non-controlling interests			0
Proceeds from non-controlling interests			0
Ending Balance	925		925
Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			2,100,297
Issuance of common stock			132,368
Common stock redemptions			(246)
Stock-based compensation			2,984
Total comprehensive loss			0
Dividends to common stockholders ($0.60 per share)			0
Ending Balance	2,235,403		2,235,403
Accumulated Other Comprehensive Loss [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			(2,092)
Issuance of common stock			0
Common stock redemptions			0
Stock-based compensation			0
Total comprehensive loss			0
Dividends to common stockholders ($0.60 per share)			0
Distributions to non-controlling interests			0
Proceeds from non-controlling interests			0
Ending Balance	(2,092)		(2,092)
Cumulative Net Income Attributable to Common Stockholders [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			801,416
Issuance of common stock			0
Common stock redemptions			0
Stock-based compensation			0
Total comprehensive loss			(25,204)
Dividends to common stockholders ($0.60 per share)			0
Distributions to non-controlling interests			0
Proceeds from non-controlling interests			0
Ending Balance	776,212		776,212
Cumulative Dividends [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			(1,779,552)
Issuance of common stock			0
Common stock redemptions			0
Stock-based compensation			0
Total comprehensive loss			0
Dividends to common stockholders ($0.60 per share)			(54,061)
Distributions to non-controlling interests			0
Proceeds from non-controlling interests			0
Ending Balance	(1,833,613)		(1,833,613)
Total Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			1,120,944
Issuance of common stock			132,417
Common stock redemptions			(246)
Stock-based compensation			2,985
Total comprehensive loss			(25,204)
Dividends to common stockholders ($0.60 per share)			(54,061)
Distributions to non-controlling interests			0
Proceeds from non-controlling interests			0
Ending Balance	1,176,835		1,176,835
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2012			0
Issuance of common stock			0
Common stock redemptions			0
Stock-based compensation			0
Total comprehensive loss			(52)
Dividends to common stockholders ($0.60 per share)			0
Distributions to non-controlling interests			(3)
Proceeds from non-controlling interests			1,388
Ending Balance	$ 1,333		$ 1,333