CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	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					$ (9,819)
Net income	$ 22,251
Other comprehensive income	232
BALANCE at Sep. 30, 2016					(9,587)
BALANCE at Dec. 31, 2016	528,468	$ 483	$ 604,364	$ (25,557)	(10,666)	$ (40,158)	$ 2
BALANCE (in shares) at Dec. 31, 2016		48,270,990				1,357,863	200,000
Cumulative effect of recording excess tax benefits from share-based payment arrangements	26,450			26,450
Net income	71,072			71,072
Other comprehensive income	(793)				(793)
Cash dividend	(4,240)			(4,240)
Exercise of common stock options	29		29
Exercise of common stock options (in shares)		1,750
Issuance of common stock to employees	8,480	$ 2	8,478
Issuance of common stock to employees (in shares)		263,154
Shares withheld for employee taxes	(9,330)		(9,330)
Shares withheld for employee taxes (in shares)		(108,732)
Issuance of common stock to Board of Directors and Scientific Advisory Board	2,190		2,190
Issuance of common stock to Board of Directors and Scientific Advisory Board (in shares)		31,064
Issuance of common stock to employees under an ESPP	684		684
Issuance of common stock to employees under an ESPP (in shares)		7,861
BALANCE at Sep. 30, 2017	$ 623,010	$ 485	$ 606,415	$ 67,725	$ (11,459)	$ (40,158)	$ 2
BALANCE (in shares) at Sep. 30, 2017		48,466,087				1,357,863,000	200,000