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	0	46,062	0	46,062
Other comprehensive loss	0	0	(3,518)	(3,518)
Dividends and dividend equivalents declared	$ 0	(27,190)	0	(27,190)
Vesting of restricted stock units (in shares)	649
Vesting of restricted stock units	$ 8,709	0	0	8,709
Stock-based compensation expense	$ 30,950	0	0	30,950
Exercise of stock options (in shares)	4
Exercise of stock options	$ 59	0	0	59
Shares issued under the employee stock purchase plan (in shares)	18
Shares issued under the employee stock purchase plan	$ 1,562	0	0	1,562
Cumulative effect of a change in accounting principles at Jun. 30, 2018	$ 0	379	0	379
Balance (in shares) at Jun. 30, 2018	42,285
Balance at Jun. 30, 2018	$ 417,866	162,859	(1,705)	579,020
Balance (in shares) at Mar. 31, 2018	42,145
Balance at Mar. 31, 2018	$ 401,007	152,257	5,042	558,306
Net income	0	24,206	0	24,206
Other comprehensive loss	0	0	(6,747)	(6,747)
Dividends and dividend equivalents declared	$ 0	(13,604)	0	(13,604)
Vesting of restricted stock units (in shares)	137
Vesting of restricted stock units	$ 916	0	0	916
Stock-based compensation expense	$ 15,900	0	0	15,900
Exercise of stock options (in shares)	3
Exercise of stock options	$ 43	0	0	43
Cumulative effect of a change in accounting principles at Jun. 30, 2018	$ 0	379	0	379
Balance (in shares) at Jun. 30, 2018	42,285
Balance at Jun. 30, 2018	$ 417,866	162,859	(1,705)	$ 579,020
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	0	46,874	0	46,874
Other comprehensive loss	0	0	1,405	1,405
Dividends and dividend equivalents declared	$ 0	(37,069)	0	(37,069)
Vesting of restricted stock units (in shares)	715
Vesting of restricted stock units	$ 12,471	0	0	12,471
Stock-based compensation expense	$ 38,753	0	0	38,753
Shares issued under the employee stock purchase plan (in shares)	14
Shares issued under the employee stock purchase plan	$ 1,627	0	0	$ 1,627
Balance (in shares) at Jun. 30, 2019	43,234			43,234
Balance at Jun. 30, 2019	$ 503,759	204,533	(4,138)	$ 704,154
Balance (in shares) at Mar. 31, 2019	43,033
Balance at Mar. 31, 2019	$ 478,913	202,378	(1,040)	680,251
Net income	0	20,693	0	20,693
Other comprehensive loss	0	0	(3,098)	(3,098)
Dividends and dividend equivalents declared	$ 0	(18,538)	0	(18,538)
Vesting of restricted stock units (in shares)	201
Vesting of restricted stock units	$ 2,088	0	0	2,088
Stock-based compensation expense	$ 22,758	0	0	$ 22,758
Balance (in shares) at Jun. 30, 2019	43,234			43,234
Balance at Jun. 30, 2019	$ 503,759	$ 204,533	$ (4,138)	$ 704,154