Consolidated Statements of Equity - USD ($) $ in Thousands	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Cumulative Net Income	Cumulative Dividends	Total Stockholders' Equity
Beginning Balance at Dec. 31, 2014	$ 0	$ 988	$ 2,389,830	$ (2,519)	$ 840,249	$ (2,007,494)	$ 1,221,054
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock, net of costs		25	66,886				66,911
Common stock redemption			(1,367)				(1,367)
Stock-based compensation		2	6,027				6,029
Net income					69,436		69,436
Amounts reclassified from accumulated other comprehensive loss arising from loss on defined benefit pension plan				2,519			2,519
Loss on interest rate swaps	0			(1,569)			(1,569)
Dividends to common stockholders ($1.20 per share)						(120,266)	(120,266)
Ending Balance at Dec. 31, 2015	0	1,015	2,461,376	(1,569)	909,685	(2,127,760)	1,242,747
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock, net of costs		140	450,409				450,549
Common stock redemption			(1,460)				(1,460)
Stock-based compensation		9	7,589				7,598
Net income					85,571		85,571
Loss on interest rate swaps		0		168			168
Dividends to common stockholders ($1.20 per share)		0				(131,759)	(131,759)
Ending Balance at Dec. 31, 2016	0	1,164	2,917,914	(1,401)	995,256	(2,259,519)	1,653,414
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock, net of costs		84	248,508				248,592
Common stock redemption		(1)	(3,017)				(3,018)
Stock-based compensation		4	10,024				10,028
Net income					23,092		23,092
Loss on interest rate swaps				102			102
Dividends to common stockholders ($1.20 per share)						(142,327)	(142,327)
Ending Balance at Dec. 31, 2017	$ 0	$ 1,251	$ 3,173,429	$ (1,299)	$ 1,018,348	$ (2,401,846)	$ 1,789,883