Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings	Treasury Stock, at cost	Accumulated Other Comprehensive Income
Beginning Balance at Dec. 31, 2017	$ 665,865	$ 250	$ 700,125	$ 247,232	$ (302,109)	$ 20,367
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common shares	(8,553)			(8,553)
Total comprehensive income	65,884			59,985		5,899
Employee stock-based compensation	3,845		3,845
Purchase of common stock under share repurchase program	(38,589)				(38,589)
Warrants exercised	10		10
Ending Balance at Mar. 31, 2018	688,462	250	703,980	298,664	(340,698)	26,266
Beginning Balance at Dec. 31, 2017	665,865	250	700,125	247,232	(302,109)	20,367
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income	226,821
Ending Balance at Sep. 30, 2018	687,140	250	712,295	449,122	(495,232)	20,705
Beginning Balance at Mar. 31, 2018	688,462	250	703,980	298,664	(340,698)	26,266
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common shares	(8,217)			(8,217)
Total comprehensive income	34,843			43,306		(8,463)
Employee stock-based compensation	4,147		4,147
Purchase of common stock under share repurchase program	(78,287)				(78,287)
Warrants exercised	0		0
Ending Balance at Jun. 30, 2018	640,948	250	708,127	333,753	(418,985)	17,803
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common shares	(7,823)			(7,823)
Total comprehensive income	126,094			123,192		2,902
Employee stock-based compensation	4,168		4,168
Purchase of common stock under share repurchase program	(76,247)				(76,247)
Ending Balance at Sep. 30, 2018	687,140	250	712,295	449,122	(495,232)	20,705
Beginning Balance at Dec. 31, 2018	704,821	250	717,492	527,666	(583,883)	43,296
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common shares	(8,111)			(8,111)
Total comprehensive income	75,835			72,741		3,094
Employee stock-based compensation	5,651		5,651
Purchase of common stock under share repurchase program	(78,249)				(78,249)
Ending Balance at Mar. 31, 2019	699,947	250	723,143	592,296	(662,132)	46,390
Beginning Balance at Dec. 31, 2018	704,821	250	717,492	527,666	(583,883)	43,296
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income	239,365
Ending Balance at Sep. 30, 2019	705,436	250	734,829	746,928	(816,882)	40,311
Beginning Balance at Mar. 31, 2019	699,947	250	723,143	592,296	(662,132)	46,390
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common shares	(7,696)			(7,696)
Total comprehensive income	63,238			62,840		398
Employee stock-based compensation	5,822		5,822
Purchase of common stock under share repurchase program	(63,392)				(63,392)
Warrants exercised	31		31
Ending Balance at Jun. 30, 2019	697,950	250	728,996	647,440	(725,524)	46,788
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends on common shares	(7,281)			(7,281)
Total comprehensive income	100,292			106,769		(6,477)
Employee stock-based compensation	5,833		5,833
Purchase of common stock under share repurchase program	(91,358)				(91,358)
Ending Balance at Sep. 30, 2019	$ 705,436	$ 250	$ 734,829	$ 746,928	$ (816,882)	$ 40,311