CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($)	Total	Cumulative Effect Period Of Adoption Adjustment [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect Period Of Adoption Adjustment [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		$ (17,100,000)				$ (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
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
Net income	138,304,000				138,304,000
Other comprehensive income (loss)	(763,000)						(763,000)
Cash dividend	(18,853,000)				(18,853,000)
Issuance of common stock to employees	15,877,000		$ 2,000	15,875,000
Issuance of common stock to employees (in shares)			247,776
Shares withheld for employee taxes	(15,980,000)			(15,980,000)
Shares withheld for employee taxes (in shares)			(99,099)
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	$ 1,848,000			1,848,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)			14,500
Issuance of common stock to employees under an ESPP	1,159,000			1,159,000
Issuance of common stock to employees under an ESPP (in shares)			7,492
BALANCE at Dec. 31, 2019	811,449,000		$ 489,000	620,236,000	249,003,000		(16,997,000)	$ (41,284,000)	$ 2,000
BALANCE (in shares) at Dec. 31, 2019			48,852,193					1,365,648	200,000
Net income	133,372,000				133,372,000
Other comprehensive income (loss)	(19,022,000)						(19,022,000)
Cash dividend	(28,445,000)				(28,445,000)
Issuance of common stock to employees	26,284,000		$ 2,000	26,282,000
Issuance of common stock to employees (in shares)			240,414
Shares withheld for employee taxes	$ (14,394,000)		$ (1,000)	(14,393,000)
Shares withheld for employee taxes (in shares)			(99,319)
Common shares repurchased (in shares)	0
Issuance of common stock to Board of Directors and Scientific Advisory Board	$ 1,947,000			1,947,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)			10,520
Issuance of common stock to employees under an ESPP	1,523,000			1,523,000
Issuance of common stock to employees under an ESPP (in shares)			9,668
BALANCE at Dec. 31, 2020	$ 912,714,000		$ 490,000	$ 635,595,000	$ 353,930,000		$ (36,019,000)	$ (41,284,000)	$ 2,000
BALANCE (in shares) at Dec. 31, 2020			49,013,476					1,365,648	200,000