Consolidated Statement of Shareholders' Equity Statement - USD ($) $ in Thousands	Total	Common Stock [Member]	Paid-in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income [Member]
Balances at Dec. 31, 2017	$ 544,020	$ 460	$ 823,850	$ (262,960)	$ (17,330)
Net income	24,320			24,320
Other comprehensive income (loss)	(1,480)				(1,480)
Shares surrendered upon exercise and vesting of equity awards to cover taxes	(2,300)		(2,300)
Non-cash compensation expense	1,220		1,220
Balances at Mar. 31, 2018	565,780	460	822,770	(238,640)	(18,810)
Balances at Dec. 31, 2017	544,020	460	823,850	(262,960)	(17,330)
Net income	66,590
Other comprehensive income (loss)	(1,550)				(1,550)
Shares surrendered upon exercise and vesting of equity awards to cover taxes	(2,380)
Balances at Sep. 30, 2018	607,490	460	822,280	(196,370)	(18,880)
Balances at Mar. 31, 2018	565,780	460	822,770	(238,640)	(18,810)
Net income	19,600			19,600
Other comprehensive income (loss)	1,910				1,910
Purchase of common stock	(2,920)		(2,920)
Shares surrendered upon exercise and vesting of equity awards to cover taxes	(80)		(80)
Non-cash compensation expense	1,400		1,400
Balances at Jun. 30, 2018	585,690	460	821,170	(219,040)	(16,900)
Net income	22,670			22,670
Other comprehensive income (loss)	(1,980)				(1,980)
Purchase of common stock	(670)		(670)
Non-cash compensation expense	1,780		1,780
Balances at Sep. 30, 2018	607,490	460	822,280	(196,370)	(18,880)
Balances at Dec. 31, 2018	620,450	460	816,500	(179,660)	(16,850)
Net income	19,090			19,090
Other comprehensive income (loss)	3,020				3,020
Purchase of common stock	(670)		(670)
Shares surrendered upon exercise and vesting of equity awards to cover taxes	(2,620)		(2,620)
Non-cash compensation expense	1,320		1,320
Impact of accounting standards adoption	(80)			1,190	(1,270)
Balances at Mar. 31, 2019	640,510	460	814,530	(159,380)	(15,100)
Balances at Dec. 31, 2018	620,450	460	816,500	(179,660)	(16,850)
Net income	60,220
Other comprehensive income (loss)	1,670				1,670
Shares surrendered upon exercise and vesting of equity awards to cover taxes	(3,240)
Balances at Sep. 30, 2019	662,060	450	796,310	(118,250)	(16,450)
Balances at Mar. 31, 2019	640,510	460	814,530	(159,380)	(15,100)
Net income	22,020			22,020
Other comprehensive income (loss)	(1,530)				(1,530)
Purchase of common stock	(14,750)	(10)	(14,740)
Shares surrendered upon exercise and vesting of equity awards to cover taxes	(610)		(610)
Non-cash compensation expense	1,720		1,720
Balances at Jun. 30, 2019	647,360	450	800,900	(137,360)	(16,630)
Net income	19,110			19,110
Other comprehensive income (loss)	180				180
Purchase of common stock	(5,670)		(5,670)
Shares surrendered upon exercise and vesting of equity awards to cover taxes	(10)		(10)
Non-cash compensation expense	1,090		1,090
Balances at Sep. 30, 2019	$ 662,060	$ 450	$ 796,310	$ (118,250)	$ (16,450)