Stockholders' Equity - Reconciliation of total stockholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Increase (Decrease) in Stockholders' Equity [Rollforward]
Net income	$ 21,300	$ 23,298
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2015	1,015
Issuance of common stock	49
Common stock redemptions	0
Stock-based compensation	3
Net income	0
Reclassification of loss on forward starting interest rate swaps	0
Dividends to common stockholders ($0.60 per share)	0
Balance at June 30, 2016	1,067
Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2015	2,461,376
Issuance of common stock	145,119
Common stock redemptions	(410)
Stock-based compensation	3,795
Net income	0
Reclassification of loss on forward starting interest rate swaps	0
Dividends to common stockholders ($0.60 per share)	0
Balance at June 30, 2016	2,609,880
Accumulated Other Comprehensive Loss [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2015	(1,569)
Issuance of common stock	0
Common stock redemptions	0
Stock-based compensation	0
Net income	0
Reclassification of loss on forward starting interest rate swaps	84
Dividends to common stockholders ($0.60 per share)	0
Balance at June 30, 2016	(1,485)
Cumulative Net Income Attributable to Common Stockholders [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2015	909,685
Issuance of common stock	0
Common stock redemptions	0
Stock-based compensation	0
Net income	21,300
Reclassification of loss on forward starting interest rate swaps	0
Dividends to common stockholders ($0.60 per share)	0
Balance at June 30, 2016	930,985
Cumulative Dividends [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2015	(2,127,760)
Issuance of common stock	0
Common stock redemptions	0
Stock-based compensation	0
Net income	0
Reclassification of loss on forward starting interest rate swaps	0
Dividends to common stockholders ($0.60 per share)	(62,239)
Balance at June 30, 2016	$ (2,189,999)
Dividends per share paid to common stockholders (in dollars per share)	$ 0.60
Total Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2015	$ 1,242,747
Issuance of common stock	145,168
Common stock redemptions	(410)
Stock-based compensation	3,798
Net income	21,300
Reclassification of loss on forward starting interest rate swaps	84
Dividends to common stockholders ($0.60 per share)	(62,239)
Balance at June 30, 2016	$ 1,350,448