Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Common Stock Including Additional Paid in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2017	41,614
Balance at Dec. 31, 2017	$ 376,586	$ 143,608	$ 1,813	$ 522,007
Net income		21,856		21,856
Other comprehensive gain (loss)			3,229	3,229
Dividends and dividend equivalents declared		(13,586)		(13,586)
Vesting of restricted stock units (in shares)	512
Vesting of restricted stock units	$ 7,793			7,793
Shares issued under the employee stock purchase plan (in shares)	18
Shares issued under the employee stock purchase plan	$ 1,563			1,563
Stock-based compensation expense	$ 15,049			15,049
Exercise of stock options (in shares)	1
Exercise of stock options	$ 16			16
Cumulative effect of a change in accounting principles at Dec. 31, 2017		379		379
Balance (in shares) at Mar. 31, 2018	42,145
Balance at Mar. 31, 2018	$ 401,007	152,257	5,042	$ 558,306
Balance (in shares) at Dec. 31, 2018	42,505			42,505
Balance at Dec. 31, 2018	$ 450,908	194,728	(5,543)	$ 640,093
Net income		26,181		26,181
Other comprehensive gain (loss)			4,503	4,503
Dividends and dividend equivalents declared		(18,531)		(18,531)
Vesting of restricted stock units (in shares)	514
Vesting of restricted stock units	$ 10,383			10,383
Shares issued under the employee stock purchase plan (in shares)	14
Shares issued under the employee stock purchase plan	$ 1,627			1,627
Stock-based compensation expense	$ 15,995			$ 15,995
Balance (in shares) at Mar. 31, 2019	43,033			43,033
Balance at Mar. 31, 2019	$ 478,913	$ 202,378	$ (1,040)	$ 680,251