Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Balance at Dec. 31, 2014	$ 1,481,500	$ 1,168	$ 1,748,303	$ (267,971)
Balance (in shares) at Dec. 31, 2014		116,764
Net income	221,854			221,854
Retirement of common stock	(9,454)	$ (3)	(9,451)
Retirement of common stock (in shares)		(237)
Dividends declared on common stock	(254,774)			(254,774)
Restricted stock compensation, net of forfeitures	14,712		14,639	73
Restricted stock compensation, net of forfeitures (in shares)		(11)
Stock option compensation expense, net of forfeitures	682		682
Income tax benefit of equity compensation	525		525
Restricted stock grants	3	$ 3
Restricted stock grants (in shares)		303
Stock options exercised	7,700	$ 4	7,696
Stock options exercised (in shares)		413
Balance at Dec. 31, 2015	1,462,748	$ 1,172	1,762,394	(300,818)
Balance (in shares) at Dec. 31, 2015		117,232
Net income	219,919			219,919
Retirement of common stock	(4,006)	$ (1)	(4,005)
Retirement of common stock (in shares)		(135)
Dividends declared on common stock	(241,721)			(241,721)
Restricted stock compensation, net of forfeitures	17,792		17,735	57
Restricted stock compensation, net of forfeitures (in shares)		(1)
Stock option compensation expense, net of forfeitures	111		111
Income tax benefit of equity compensation	1,479		1,479
Restricted stock grants	3	$ 3
Restricted stock grants (in shares)		318
Stock options exercised	2,638	$ 2	2,636
Stock options exercised (in shares)		140
Balance at Dec. 31, 2016	1,458,963	$ 1,176	1,780,350	(322,563)
Balance (in shares) at Dec. 31, 2016		117,554
Net income	178,040			178,040
Retirement of common stock	(5,847)	$ (2)	(5,845)
Retirement of common stock (in shares)		(176)
Dividends declared on common stock	(199,764)			(199,764)
Restricted stock compensation, net of forfeitures	13,286		13,286
Restricted stock grants		$ 5	(5)
Restricted stock grants (in shares)		513
Stock options exercised	6,930	$ 3	6,927
Stock options exercised (in shares)		313
Balance at Dec. 31, 2017	$ 1,451,608	$ 1,182	$ 1,794,713	$ (344,287)
Balance (in shares) at Dec. 31, 2017		118,204