Stockholders' Equity - Reconciliation of total stockholders' equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Increase (Decrease) in Stockholders' Equity [Rollforward]
Net income	$ 27,480	$ 4,153	$ 50,778	$ 14,127
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan	0	$ 0	2,519	$ 0
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			988
Issuance of common stock			16
Stock-based compensation			1
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			0
Balance at September 30, 2015	1,005		1,005
Additional Paid-In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			2,389,830
Issuance of common stock			41,733
Common stock redemptions			(271)
Stock-based compensation			4,557
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			0
Balance at September 30, 2015	2,435,849		2,435,849
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			(1,611)
Balance at September 30, 2015	(1,611)		(1,611)
Cumulative Net Income Attributable to Common Stockholders [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			840,249
Net income			50,778
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			0
Balance at September 30, 2015	891,027		891,027
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 ($0.90 per share)			(90,078)
Balance at September 30, 2015	(2,097,572)		(2,097,572)
Total Stockholders' Equity [Member]
Increase (Decrease) in Stockholders' Equity [Rollforward]
Balance at December 31, 2014			1,221,054
Issuance of common stock			41,749
Common stock redemptions			(271)
Stock-based compensation			4,558
Net income			50,778
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan			2,519
Loss on forward starting interest rate swaps			(1,611)
Dividends to common stockholders ($0.90 per share)			(90,078)
Balance at September 30, 2015	$ 1,228,698		$ 1,228,698