Consolidated Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (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, 2016	1,458,963	$ 1,176	1,780,350	(322,563)
Balance (in shares) at Dec. 31, 2016		117,554
Comprehensive income
Net income	136,700
Balance at Sep. 30, 2017	1,458,681	$ 1,182	1,793,568	(336,069)
Balance (in shares) at Sep. 30, 2017		118,191
Balance at Mar. 31, 2017	1,463,161	$ 1,181	1,784,532	(322,552)
Balance (in shares) at Mar. 31, 2017		118,140
Comprehensive income
Net income	45,475			45,475
Retirement of common stock	(729)		(729)
Retirement of common stock (in shares)		(22)
Dividends declared on common stock	(50,098)			(50,098)
Restricted stock compensation, net of forfeitures	4,059		4,059
Restricted stock grants (in shares)		1
Stock options exercised	1,476	$ 1	1,475
Stock options exercised (in shares)		60
Balance at Jun. 30, 2017	1,463,344	$ 1,182	1,789,337	(327,175)
Balance (in shares) at Jun. 30, 2017		118,179
Comprehensive income
Net income	41,178			41,178
Dividends declared on common stock	(50,072)			(50,072)
Restricted stock compensation, net of forfeitures	4,058		4,058
Restricted stock grants (in shares)		2
Stock options exercised	173		173
Stock options exercised (in shares)		10
Balance at Sep. 30, 2017	1,458,681	$ 1,182	1,793,568	(336,069)
Balance (in shares) at Sep. 30, 2017		118,191
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 compensation, net of forfeitures (in shares)
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
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	117,968
Balance at Sep. 30, 2018	1,422,075	$ 1,187	1,803,903	(383,015)
Balance (in shares) at Sep. 30, 2018		118,670
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
Comprehensive income
Net income	39,197			39,197
Change in fair value of interest rate swap, net of tax	2,028				2,028
Total comprehensive income	41,225			39,197	2,028
Retirement of common stock	(19)		(19)
Retirement of common stock (in shares)		(1)
Dividends declared on common stock	(51,478)			(51,478)
Restricted stock compensation, net of forfeitures	3,980		3,980
Restricted stock compensation, net of forfeitures (in shares)
Restricted stock grants
Restricted stock grants (in shares)		5
Balance at Jun. 30, 2018	1,427,918	$ 1,185	1,799,632	(372,899)
Balance (in shares) at Jun. 30, 2018		118,548
Comprehensive income
Net income	40,994	$ 0	0	40,994	0
Dividends declared on common stock	(51,110)	0	0	(51,110)	0
Restricted stock compensation, net of forfeitures	2,292	$ 0	2,292	0	0
Restricted stock compensation, net of forfeitures (in shares)		0
Stock options exercised	1,981	$ 2	1,979	0	0
Stock options exercised (in shares)		122
Balance at Sep. 30, 2018	$ 1,422,075	$ 1,187	$ 1,803,903	$ (383,015)
Balance (in shares) at Sep. 30, 2018		118,670