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	77,630	0	77,630
Other comprehensive loss	0	0	(7,553)	(7,553)
Dividends and dividend equivalents declared	$ 0	(40,798)	0	(40,798)
Common stock issued under employee equity incentive plans (in shares)	761
Common stock issued under employee equity incentive plans	$ 9,684	0	0	9,684
Common stock issued under employee stock purchase plan (in shares)	33
Common stock issued under employee stock purchase plan	$ 3,028	0	0	3,028
Stock-based compensation expense	45,787	0	0	45,787
Cumulative effect of a change in accounting principles at Sep. 30, 2018	$ 0	379	0	379
Balance (in shares) at Sep. 30, 2018	42,408
Balance at Sep. 30, 2018	$ 435,085	180,819	(5,740)	610,164
Balance (in shares) at Jun. 30, 2018	42,285
Balance at Jun. 30, 2018	$ 417,866	162,859	(1,705)	579,020
Net income	0	31,568	0	31,568
Other comprehensive loss	0	0	(4,035)	(4,035)
Dividends and dividend equivalents declared	$ 0	(13,608)	0	(13,608)
Common stock issued under employee equity incentive plans (in shares)	108
Common stock issued under employee equity incentive plans	$ 1,465	0	0	1,465
Common stock issued under employee stock purchase plan (in shares)	15
Common stock issued under employee stock purchase plan	$ 916	0	0	916
Stock-based compensation expense	14,838	0	0	14,838
Cumulative effect of a change in accounting principles at Sep. 30, 2018	$ 0	379	0	379
Balance (in shares) at Sep. 30, 2018	42,408
Balance at Sep. 30, 2018	$ 435,085	180,819	(5,740)	$ 610,164
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	76,402	0	76,402
Other comprehensive loss	0	0	(4,496)	(4,496)
Dividends and dividend equivalents declared	$ 0	(55,438)	0	(55,438)
Common stock issued under employee equity incentive plans (in shares)	902
Common stock issued under employee equity incentive plans	$ 14,561	0	0	14,561
Common stock issued under employee stock purchase plan (in shares)	28
Common stock issued under employee stock purchase plan	$ 3,277	0	0	3,277
Stock-based compensation expense	$ 60,029	0	0	$ 60,029
Balance (in shares) at Sep. 30, 2019	43,435			43,435
Balance at Sep. 30, 2019	$ 528,775	215,692	(10,039)	$ 734,428
Balance (in shares) at Jun. 30, 2019	43,234
Balance at Jun. 30, 2019	$ 503,759	204,533	(4,138)	704,154
Net income	0	29,528	0	29,528
Other comprehensive loss	0	0	(5,901)	(5,901)
Dividends and dividend equivalents declared	$ 0	(18,369)	0	(18,369)
Common stock issued under employee equity incentive plans (in shares)	187
Common stock issued under employee equity incentive plans	$ 2,090	0	0	2,090
Common stock issued under employee stock purchase plan (in shares)	14
Common stock issued under employee stock purchase plan	$ 1,650	0	0	1,650
Stock-based compensation expense	$ 21,276	0	0	$ 21,276
Balance (in shares) at Sep. 30, 2019	43,435			43,435
Balance at Sep. 30, 2019	$ 528,775	$ 215,692	$ (10,039)	$ 734,428