Consolidated Statements of Stockholders' Equity - 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, 2015	39,689
Balance at Dec. 31, 2015	$ 265,763	$ 101,287	$ 1,466	$ 368,516
Net income		52,720		52,720
Other comprehensive gain (loss)			(5,681)	(5,681)
Dividends and dividend equivalents declared		(34,645)		$ (34,645)
Exercise of stock options (in shares)	76			76
Exercise of stock options	$ 1,344			$ 1,344
Vesting of restricted stock units (in shares)	975
Vesting of restricted stock units
Shares issued under the employee stock purchase plan (in shares)	53
Shares issued under the employee stock purchase plan	$ 2,463			2,463
Stock-based compensation expense	44,934			44,934
Tax benefits from equity awards	$ 1,465			1,465
Balance (in shares) at Dec. 31, 2016	40,793
Balance at Dec. 31, 2016	$ 315,969	119,362	(4,215)	431,116
Net income		65,203		65,203
Other comprehensive gain (loss)			6,028	6,028
Dividends and dividend equivalents declared		(35,816)		$ (35,816)
Exercise of stock options (in shares)	9			9
Exercise of stock options	$ 150			$ 150
Vesting of restricted stock units (in shares)	772
Vesting of restricted stock units
Shares issued under the employee stock purchase plan (in shares)	40
Shares issued under the employee stock purchase plan	$ 2,701			2,701
Stock-based compensation expense	$ 52,625			$ 52,625
Balance (in shares) at Dec. 31, 2017	41,614			41,614
Balance at Dec. 31, 2017	$ 376,586	143,608	1,813	$ 522,007
Cumulative effect of a change in accounting principles	$ 5,141	(5,141)
Net income		105,268		105,268
Other comprehensive gain (loss)			(7,356)	(7,356)
Dividends and dividend equivalents declared		(54,527)		$ (54,527)
Exercise of stock options (in shares)	5			5
Exercise of stock options	$ 59			$ 59
Vesting of restricted stock units (in shares)	853
Vesting of restricted stock units	$ 10,578			10,578
Shares issued under the employee stock purchase plan (in shares)	33
Shares issued under the employee stock purchase plan	$ 3,028			3,028
Stock-based compensation expense	$ 60,657			$ 60,657
Balance (in shares) at Dec. 31, 2018	42,505			42,505
Balance at Dec. 31, 2018	$ 450,908	194,728	(5,543)	$ 640,093
Cumulative effect of a change in accounting principles		$ 379		$ 379