CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($)	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Treasury Stock [Member]	Series A Nonconvertible Preferred Stock [Member] Preferred Stock [Member]
BALANCE at Dec. 31, 2015	$ 466,765,000	$ 482,000	$ 589,885,000	$ (73,627,000)	$ (9,819,000)	$ (40,158,000)	$ 2,000
BALANCE (in shares) at Dec. 31, 2015		48,132,223				1,357,863	200,000
Net income	48,070,000			48,070,000
Other comprehensive income (loss)	(847,000)				(847,000)
Exercise of common stock options	185,000		185,000
Exercise of common stock options (in shares)		12,750
Issuance of common stock to employees	12,356,000	$ 2,000	12,354,000
Issuance of common stock to employees (in shares)		165,826
Shares withheld for employee taxes	(4,871,000)	$ (1,000)	(4,870,000)
Shares withheld for employee taxes (in shares)		(92,241)
Cumulative effect of recording excess tax benefits from share-basedpayment arrangements	4,232,000		4,232,000
Issuance of common stock to Board of Directors and Scientific Advisory Board	2,012,000		2,012,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		43,046
Issuance of common stock to employees under an ESPP	566,000		566,000
Issuance of common stock to employees under an ESPP (in shares)		9,386
BALANCE at Dec. 31, 2016	528,468,000	$ 483,000	604,364,000	(25,557,000)	(10,666,000)	$ (40,158,000)	$ 2,000
BALANCE (in shares) at Dec. 31, 2016		48,270,990				1,357,863	200,000
Net income	103,885,000			103,885,000
Other comprehensive income (loss)	(798,000)				(798,000)
Cash dividend	(5,652,000)			(5,652,000)
Exercise of common stock options	38,000		38,000
Exercise of common stock options (in shares)		2,250
Issuance of common stock to employees	12,242,000	$ 3,000	12,239,000
Issuance of common stock to employees (in shares)		265,233
Shares withheld for employee taxes	(9,432,000)	$ (1,000)	(9,431,000)
Shares withheld for employee taxes (in shares)		(109,483)
Cumulative effect of recording excess tax benefits from share-basedpayment arrangements	26,450,000			26,450,000
Issuance of common stock to Board of Directors and Scientific Advisory Board	2,909,000		2,909,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		37,314
Issuance of common stock to employees under an ESPP	944,000		944,000
Issuance of common stock to employees under an ESPP (in shares)		9,730
BALANCE at Dec. 31, 2017	659,054,000	$ 485,000	611,063,000	99,126,000	(11,464,000)	$ (40,158,000)	$ 2,000
BALANCE (in shares) at Dec. 31, 2017		48,476,034				1,357,863	200,000
ASC Topic 606 Adoption at Dec. 31, 2017	(17,100,000)			(17,100,000)
ADJUSTED BALANCE at Dec. 31, 2017	641,954,000	$ 485,000	611,063,000	82,026,000	(11,464,000)	$ (40,158,000)	$ 2,000
BALANCE ADJUSTED (in shares) at Dec. 31, 2017		48,476,034				1,357,863	200,000
Net income	58,840,000			58,840,000
Other comprehensive income (loss)	(4,770,000)				(4,770,000)
Cash dividend	(11,314,000)			(11,314,000)
Issuance of common stock to employees	12,139,000	$ 3,000	12,136,000
Issuance of common stock to employees (in shares)		271,068
Shares withheld for employee taxes	(11,620,000)	$ (1,000)	(11,619,000)
Shares withheld for employee taxes (in shares)		(108,113)
Common shares repurchased	$ (477,000)					$ (477,000)
Common shares repurchased (in shares)	(3,774)					3,774
Issuance of common stock to Board of Directors and Scientific Advisory Board	$ 4,664,000		4,664,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		32,232
Issuance of common stock to employees under an ESPP	1,090,000		1,090,000
Issuance of common stock to employees under an ESPP (in shares)		10,303
BALANCE at Dec. 31, 2018	$ 690,506,000	$ 487,000	$ 617,334,000	$ 129,552,000	$ (16,234,000)	$ (40,635,000)	$ 2,000
BALANCE (in shares) at Dec. 31, 2018		48,681,524				1,361,637	200,000