CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Treasury Stock
Beginning Balance at Dec. 31, 2017	$ 941,597		$ 746,547	$ 194,971	$ 79
Beginning Balance (in shares) at Dec. 31, 2017		76,825,966
Issuance of restricted stock units	3,679		3,679
Issuance of restricted stock units (in shares)		645,047
Net income	20,855			20,855
Shares withheld related to net share settlement of stock awards	(7,502)		(7,502)
Shares withheld related to net share settlement of stock awards (in shares)		(229,898)
Ending Balance at Mar. 31, 2018	958,629		742,724	215,826	79
Ending Balance (in shares) at Mar. 31, 2018		77,241,115
Beginning Balance at Dec. 31, 2017	941,597		746,547	194,971	79
Beginning Balance (in shares) at Dec. 31, 2017		76,825,966
Net income	95,355
Ending Balance at Sep. 30, 2018	1,036,151	$ 1	745,745	290,326	79
Ending Balance (in shares) at Sep. 30, 2018		77,391,244
Beginning Balance at Mar. 31, 2018	958,629		742,724	215,826	79
Beginning Balance (in shares) at Mar. 31, 2018		77,241,115
Issuance of restricted stock units	3,472	$ 1	3,471
Issuance of restricted stock units (in shares)		64,833
Net income	36,796			36,796
Exercise of stock options (in shares)		50,336
Shares withheld related to net share settlement of stock awards	(2,175)		(2,175)
Shares withheld related to net share settlement of stock awards (in shares)		(19,674)
Ending Balance at Jun. 30, 2018	996,722	$ 1	744,020	252,622	79
Ending Balance (in shares) at Jun. 30, 2018		77,336,610
Issuance of restricted stock units	2,495		2,495
Issuance of restricted stock units (in shares)		61,535
Net income	37,704			37,704
Exercise of stock options	154		154
Exercise of stock options (in shares)		17,000
Shares withheld related to net share settlement of stock awards	(924)		(924)
Shares withheld related to net share settlement of stock awards (in shares)		(23,901)
Ending Balance at Sep. 30, 2018	1,036,151	$ 1	745,745	290,326	79
Ending Balance (in shares) at Sep. 30, 2018		77,391,244
Beginning Balance at Dec. 31, 2018	1,029,153	$ 1	748,076	290,206	1	$ (9,131)
Beginning Balance (in shares) at Dec. 31, 2018		77,438,889				223,823
Cumulative change in accounting principle, net of tax	(4,744)			(4,744)
Issuance of restricted stock units	4,948		4,948
Issuance of restricted stock units (in shares)		330,641
Net income	38,229			38,229
Shares withheld related to net share settlement of stock awards	(4,322)		(4,322)
Shares withheld related to net share settlement of stock awards (in shares)		(106,542)
Ending Balance at Mar. 31, 2019	1,063,264	$ 1	748,702	323,691	1	$ (9,131)
Ending Balance (in shares) at Mar. 31, 2019		77,662,988				223,823
Beginning Balance at Dec. 31, 2018	1,029,153	$ 1	748,076	290,206	1	$ (9,131)
Beginning Balance (in shares) at Dec. 31, 2018		77,438,889				223,823
Net income	94,220
Ending Balance at Sep. 30, 2019	1,126,778	$ 1	756,225	379,682	1	$ (9,131)
Ending Balance (in shares) at Sep. 30, 2019		77,769,501				223,823
Beginning Balance at Mar. 31, 2019	1,063,264	$ 1	748,702	323,691	1	$ (9,131)
Beginning Balance (in shares) at Mar. 31, 2019		77,662,988				223,823
Issuance of restricted stock units	6,509		6,509
Issuance of restricted stock units (in shares)		169,248
Net income	18,936			18,936
Shares withheld related to net share settlement of stock awards	(3,190)		(3,190)
Shares withheld related to net share settlement of stock awards (in shares)		(65,312)
Ending Balance at Jun. 30, 2019	1,085,519	$ 1	752,021	342,627	1	$ (9,131)
Ending Balance (in shares) at Jun. 30, 2019		77,766,924				223,823
Issuance of restricted stock units	4,266		4,266
Issuance of restricted stock units (in shares)		3,377
Net income	37,055			37,055
Shares withheld related to net share settlement of stock awards	(62)		(62)
Shares withheld related to net share settlement of stock awards (in shares)		(800)
Ending Balance at Sep. 30, 2019	$ 1,126,778	$ 1	$ 756,225	$ 379,682	$ 1	$ (9,131)
Ending Balance (in shares) at Sep. 30, 2019		77,769,501				223,823