CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($)	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss [Member]	Treasury Stock [Member]	Series A Nonconvertible Preferred Stock [Member] Preferred Stock [Member]
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	31,474,000			31,474,000
Other comprehensive income	714,000				714,000
Cash dividend	(4,717,000)			(4,717,000)
Issuance of common stock to employees	3,537,000	$ 1,000	3,536,000
Issuance of common stock to employees (in shares)		107,213
Shares withheld for employee taxes	(5,757,000)		(5,757,000)
Shares withheld for employee taxes (in shares)		(38,755)
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	$ 816,000		816,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		7,758
Issuance of common stock to employees under an ESPP	277,000		277,000
Issuance of common stock to employees under an ESPP (in shares)		2,571
BALANCE at Mar. 31, 2019	716,201,000	$ 488,000	616,206,000	156,309,000	(15,520,000)	$ (41,284,000)	$ 2,000
BALANCE (in shares) at Mar. 31, 2019		48,760,311				1,365,648	200,000
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	38,155,000			38,155,000
Other comprehensive income	2,460,000				2,460,000
Cash dividend	(7,098,000)			(7,098,000)
Issuance of common stock to employees	5,665,000		5,665,000
Issuance of common stock to employees (in shares)		80,483
Shares withheld for employee taxes	$ (4,816,000)		(4,816,000)
Shares withheld for employee taxes (in shares)		(30,437)
Common shares repurchased (in shares)	0
Issuance of common stock to Board of Directors and Scientific Advisory Board	$ 554,000		554,000
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		5,678
Issuance of common stock to employees under an ESPP	328,000		328,000
Issuance of common stock to employees under an ESPP (in shares)		2,315
BALANCE at Mar. 31, 2020	$ 846,697,000	$ 489,000	$ 621,967,000	$ 280,060,000	$ (14,537,000)	$ (41,284,000)	$ 2,000
BALANCE (in shares) at Mar. 31, 2020		48,910,232				1,365,648	200,000