Consolidated Statement of Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock Including Additional Paid in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Common Stock [Member]	Total
Balance at Dec. 31, 2017	$ 369,686	$ 366,164	$ (129,621)
Exercise of stock options	144
Stock-based compensation expense	9,494
Vesting of restricted stock units, net of shares withheld for employee tax	(2,587)
Balance at Mar. 31, 2018	376,737	373,982	(135,576)		$ 615,143
Net income		6,391			$ 6,391
Cumulative effect of accounting changes		1,427
Purchase of treasury stock			$ (5,955)
Balance, beginning of period (in shares) at Dec. 31, 2017			13,279,393	96,536,251
Exercise of stock options (in shares)				9,043	9,043
Vesting of restricted stock units, net of shares withheld for employee tax (in shares)				330,860
Balance, end of period (in shares) at Mar. 31, 2018			13,663,194	96,876,154
Purchase of treasury stock (in shares)			383,801
Balance at Dec. 31, 2018	426,737	422,235	$ (135,576)		$ 713,396
Exercise of stock options	23,139
Stock-based compensation expense	10,979
Vesting of restricted stock units, net of shares withheld for employee tax	(6,768)
Balance at Mar. 31, 2019	$ 454,087	441,877	(135,576)		760,388
Net income		19,642			$ 19,642
Cumulative effect of accounting changes
Purchase of treasury stock
Balance, beginning of period (in shares) at Dec. 31, 2018			13,663,194	98,924,501
Exercise of stock options (in shares)				1,443,685	1,443,685
Vesting of restricted stock units, net of shares withheld for employee tax (in shares)				376,891
Balance, end of period (in shares) at Mar. 31, 2019			13,663,194	100,745,077
Purchase of treasury stock (in shares)