Consolidated Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2016	$ 1,458,963	$ 1,176	$ 1,780,350	$ (322,563)
Balance (in shares) at Dec. 31, 2016		117,554
Comprehensive income
Net income	50,047			50,047
Retirement of common stock	(5,089)	$ (2)	(5,087)
Retirement of common stock (in shares)		(153)
Dividends declared on common stock	(50,036)			(50,036)
Restricted stock compensation, net of forfeitures	4,086		4,086
Restricted stock grants		$ 5	(5)
Restricted stock grants (in shares)		506
Stock options exercised	5,190	$ 2	5,188
Stock options exercised (in shares)		233
Balance at Mar. 31, 2017	1,463,161	$ 1,181	1,784,532	(322,552)
Balance (in shares) at Mar. 31, 2017		118,140
Balance at Dec. 31, 2017	1,451,608	$ 1,182	1,794,713	(344,287)
Balance (in shares) at Dec. 31, 2017		118,204
Comprehensive income
Net income	37,777			37,777
Change in fair value of interest rate swap, net of tax	(2,028)				$ (2,028)
Total comprehensive income	35,749			37,777	(2,028)
Retirement of common stock	(2,525)	$ (1)	(2,524)
Retirement of common stock (in shares)		(117)
Dividends declared on common stock	(51,533)			(51,533)
Restricted stock compensation, net of forfeitures	3,486		3,486
Restricted stock grants		$ 4	(4)
Restricted stock grants (in shares)		457
Cumulative effect of adoption of new accounting standard	(2,575)			(2,575)
Balance at Mar. 31, 2018	$ 1,434,210	$ 1,185	$ 1,795,671	$ (360,618)	$ (2,028)
Balance (in shares) at Mar. 31, 2018		118,544