CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (UNAUDITED) - 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, 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	$ (17,894,000)			(17,894,000)
ADJUSTED BALANCE at Dec. 31, 2017	$ 641,160,000	485,000	611,063,000	81,232,000	(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	$ 39,591,000			39,591,000
Other comprehensive income	1,470,000				1,470,000
Cash dividend	(8,486,000)			(8,486,000)
Issuance of common stock to employees	$ 9,457,000	3,000	9,454,000
Issuance of common stock to employees (in shares)	270,451
Shares withheld for employee taxes	$ (11,598,000)	(1,000)	(11,597,000)
Shares withheld for employee taxes (in shares)	(107,920)
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	$ 3,534,000		3,534,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)	25,982
Issuance of common stock to employees under an ESPP	$ 856,000		856,000
Issuance of common stock to employees under an ESPP (in shares)	7,991
BALANCE at Sep. 30, 2018	$ 675,507,000	487,000	613,310,000	112,337,000	(9,994,000)	$ (40,635,000)	$ 2,000
BALANCE (in shares) at Sep. 30, 2018	48,672,538					1,361,637	200,000
BALANCE at Jun. 30, 2018	$ 650,681,000	487,000	608,932,000	92,346,000	(10,451,000)	$ (40,635,000)	$ 2,000
BALANCE (in shares) at Jun. 30, 2018	48,650,696					1,361,637	200,000
Net income	$ 22,818,000			22,818,000
Other comprehensive income	457,000				457,000
Cash dividend	(2,827,000)			(2,827,000)
Issuance of common stock to employees	$ 3,422,000		3,422,000
Issuance of common stock to employees (in shares)	19,472
Shares withheld for employee taxes	$ (567,000)		(567,000)
Shares withheld for employee taxes (in shares)	(6,339)
Common shares repurchased (in shares)	0
Issuance of common stock to Board of Directors and Scientific Advisory Board	$ 1,231,000		1,231,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)	6,250
Issuance of common stock to employees under an ESPP	$ 292,000		292,000
Issuance of common stock to employees under an ESPP (in shares)	2,459
BALANCE at Sep. 30, 2018	$ 675,507,000	487,000	613,310,000	112,337,000	(9,994,000)	$ (40,635,000)	$ 2,000
BALANCE (in shares) at Sep. 30, 2018	48,672,538					1,361,637	200,000
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
Net income	111,876,000			111,876,000
Other comprehensive income	2,115,000				2,115,000
Cash dividend	(14,142,000)			(14,142,000)
Issuance of common stock to employees	10,490,000	$ 2,000	10,488,000
Issuance of common stock to employees (in shares)		246,715
Shares withheld for employee taxes	(15,920,000)	$ (1,000)	(15,919,000)
Shares withheld for employee taxes (in shares)		(98,782)
Common shares repurchased	$ (649,000)					$ (649,000)
Common shares repurchased (in shares)	4,011					4,011
Issuance of common stock to Board of Directors and Scientific Advisory Board	$ 1,426,000		1,426,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		12,424
Issuance of common stock to employees under an ESPP	865,000		865,000
Issuance of common stock to employees under an ESPP (in shares)		5,852
BALANCE at Sep. 30, 2019	786,567,000	$ 488,000	614,194,000	227,286,000	(14,119,000)	$ (41,284,000)	$ 2,000
BALANCE (in shares) at Sep. 30, 2019		48,847,733				1,365,648	200,000
BALANCE at Jun. 30, 2019	751,033,000	$ 488,000	611,596,000	195,036,000	(14,805,000)	$ (41,284,000)	$ 2,000
BALANCE (in shares) at Jun. 30, 2019		48,831,512				1,365,648	200,000
Net income	36,962,000			36,962,000
Other comprehensive income	686,000				686,000
Cash dividend	(4,712,000)			(4,712,000)
Issuance of common stock to employees	3,533,000		3,533,000
Issuance of common stock to employees (in shares)		20,069
Shares withheld for employee taxes	$ (1,448,000)		(1,448,000)
Shares withheld for employee taxes (in shares)		(7,416)
Common shares repurchased (in shares)	0
Issuance of common stock to Board of Directors and Scientific Advisory Board	$ 242,000		242,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		2,076
Issuance of common stock to employees under an ESPP	271,000		271,000
Issuance of common stock to employees under an ESPP (in shares)		1,492
BALANCE at Sep. 30, 2019	$ 786,567,000	$ 488,000	$ 614,194,000	$ 227,286,000	$ (14,119,000)	$ (41,284,000)	$ 2,000
BALANCE (in shares) at Sep. 30, 2019		48,847,733				1,365,648	200,000