Stockholders' Equity - Reconciliation of total stockholders' equity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Increase (Decrease) in Stockholders' Equity [Rollforward]
Net income	$ 17,916	$ 6,011	$ 23,298	$ 9,974
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan	2,519	$ 0	2,519	$ 0
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			988
Issuance of common stock			15
Stock-based compensation			1
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			0
Balance at June 30, 2015	1,004		1,004
Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			2,389,830
Issuance of common stock			40,343
Common stock redemptions			(271)
Stock-based compensation			3,077
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			0
Balance at June 30, 2015	2,432,979		2,432,979
Accumulated Other Comprehensive Loss [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			(2,519)
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			2,519
Loss on forward starting interest rate swaps			(1,653)
Balance at June 30, 2015	(1,653)		(1,653)
Cumulative Net Income Attributable to Common Stockholders [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			840,249
Net income			23,298
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			0
Balance at June 30, 2015	863,547		863,547
Cumulative Dividends [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			(2,007,494)
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			0
Dividends to common stockholders ($0.60 per share)			(59,954)
Balance at June 30, 2015	(2,067,448)		(2,067,448)
Total Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			1,221,054
Issuance of common stock			40,358
Common stock redemptions			(271)
Stock-based compensation			3,078
Net income			23,298
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			2,519
Loss on forward starting interest rate swaps			(1,653)
Dividends to common stockholders ($0.60 per share)			(59,954)
Balance at June 30, 2015	$ 1,228,429		$ 1,228,429