Consolidated Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in-Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning balance at Dec. 31, 2014	$ 978,010	$ 257	$ 286,854	$ 852,057	$ (81,984)	$ (79,174)
Beginning balance, shares at Dec. 31, 2014		25,724
Net income	93,999	$ 0	0		0	0
Other comprehensive income (loss), net	(56,000)	0	0	0	(56,000)	0
Stock Issued During Period, Value, Treasury Stock Reissued		$ 0	0	0	0	99,015
Stock Issued During Period, Shares, Employee Stock Purchase Plans		(18)
Stock Issued During Period, Value, Employee Stock Purchase Plan	206	$ 0	(16,701)	0	0	16,907
Purchases of treasury stock	(99,015)
Stock-based compensation	20,939	0	20,939	0	0	0
Cash dividends	(19,887)	0	0	(19,887)	0	0
Ending balance at Dec. 31, 2015	918,252	$ 257	291,092	926,169	(137,984)	(161,282)
Ending balance, shares at Dec. 31, 2015		25,706
Net income	147,065	$ 0	0		0	0
Other comprehensive income (loss), net	(8,960)	0	0	0	(8,960)	0
Stock Issued During Period, Value, Treasury Stock Reissued		$ 0	0	0	0	129,187
Stock Issued During Period, Shares, Employee Stock Purchase Plans		(43)
Stock Issued During Period, Value, Employee Stock Purchase Plan	2,193	$ 0	(20,963)	0	0	23,156
Purchases of treasury stock	(129,187)
Stock-based compensation	20,770	0	20,770	0	0	0
Cash dividends	(19,933)	0	0	(19,933)	0	0
Ending balance at Dec. 31, 2016	930,200	$ 257	290,899	1,053,301	(146,944)	(267,313)
Ending balance, shares at Dec. 31, 2016		25,663
Net income	213,442	$ 0	0		0	0
Other comprehensive income (loss), net	23,718	0	0	0	23,718	0
Stock Issued During Period, Value, Treasury Stock Reissued		$ 0	0	0	0	(42,084)
Stock Issued During Period, Shares, Employee Stock Purchase Plans		(148)
Stock Issued During Period, Value, Employee Stock Purchase Plan	571	$ (2)	(18,293)	0	0	18,866
Purchases of treasury stock	(42,084)
Stock-based compensation	18,855	0	18,855	0	0	0
Cash dividends	(20,420)	0	0	(20,420)	0	0
Ending balance at Dec. 31, 2017	$ 1,124,282	$ 255	$ 291,461	$ 1,246,323	$ (123,226)	$ (290,531)
Ending balance, shares at Dec. 31, 2017		25,515