Consolidated Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
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	95,522			95,522
Retirement of common stock	(5,818)	$ (2)	(5,816)
Retirement of common stock (in shares)		(175)
Dividends declared on common stock	(100,134)			(100,134)
Restricted stock compensation, net of forfeitures	8,145		8,145
Restricted stock grants		$ 5	(5)
Restricted stock grants (in shares)		507
Stock options exercised	6,666	$ 3	6,663
Stock options exercised (in shares)		293
Balance at Jun. 30, 2017	1,463,344	$ 1,182	1,789,337	(327,175)
Balance (in shares) at Jun. 30, 2017		118,179
Balance at Dec. 31, 2017	1,451,608	$ 1,182	1,794,713	(344,287)
Balance (in shares) at Dec. 31, 2017		118,204
Net income	76,974			76,974
Retirement of common stock	(2,544)	$ (1)	(2,543)
Retirement of common stock (in shares)		(118)
Dividends declared on common stock	(103,011)			(103,011)
Restricted stock compensation, net of forfeitures	7,466		7,466
Restricted stock grants		$ 4	(4)
Restricted stock grants (in shares)		462
Cumulative effect of adoption of new accounting standard	(2,575)			(2,575)
Balance at Jun. 30, 2018	$ 1,427,918	$ 1,185	$ 1,799,632	$ (372,899)
Balance (in shares) at Jun. 30, 2018		118,548