Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
BALANCE at Dec. 31, 2014	$ 833,860	$ 805	$ 101,803	$ 915,085	$ (9,375)	$ (174,458)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	120,026	0	0	123,714	(3,688)	0
Purchases of common stock	(6,438)	0	0	0	0	(6,438)
Dividends on common stock	(15,833)	0	0	(15,833)	0	0
Equity compensation activity, including excess tax benefits	(322)	0	(3,430)	0	0	3,108
Non-cash equity compensation expense	4,361	0	4,361	0	0	0
BALANCE at Dec. 31, 2015	935,654	805	102,734	1,022,966	(13,063)	(177,788)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	75,275	0	0	79,129	(3,854)	0
Dividends on common stock	(17,299)	0	0	(17,299)	0	0
Equity compensation activity, including excess tax benefits	(1,224)	0	(4,080)	0	0	2,856
Non-cash equity compensation expense	2,381	0	2,381	0	0	0
BALANCE at Dec. 31, 2016	994,787	805	101,035	1,084,796	(16,917)	(174,932)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	203,971	0	0	202,889	1,082	0
Dividends on common stock	(19,523)	0	0	(19,523)	0	0
Equity compensation activity, including excess tax benefits	829	0	(3,481)	0	0	4,310
Non-cash equity compensation expense	4,546	0	4,546	0	0	0
Effect of ASU 2016-09	172	0	463	(291)	0	0
BALANCE at Dec. 31, 2017	$ 1,184,782	$ 805	$ 102,563	$ 1,267,871	$ (15,835)	$ (170,622)