Consolidated Statements of Stockholders' Equity (Unaudited) - 9 months ended Sep. 30, 2018 - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Beginning Balance at Dec. 31, 2017	$ 985,951	$ 523	$ (100,694)	$ 232,670	$ (724)	$ 854,176
Beginning Balance, Shares at Dec. 31, 2017		52,277	4,152
Comprehensive income	153,708				228	153,480
Adoption impact - ASU 2018-02 | Accounting Standards Update 2018-02 [Member]					(156)	156
Common stock purchased for treasury	(4,135)		$ (4,135)
Common stock purchased for treasury, shares			39
Restricted shares forfeited	0	$ 0	$ 0	0	0	0
Restricted shares forfeited, Shares			94
Share-based compensation	1,913	$ 2	$ (15,153)	17,064
Share-based compensation, Shares		163	151
Exercise of stock options	$ 2,095	$ 1		2,094
Exercise of stock options, Shares	130	130
Ending Balance at Sep. 30, 2018	$ 1,138,358	$ 526	$ (119,982)	$ 251,828	$ (652)	1,006,638
Ending Balance, Shares at Sep. 30, 2018		52,570	4,436
Cumulative effect from the adoption of accounting pronouncements, net of taxes	$ (1,174)					$ (1,174)