CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2015	$ 524,938	$ 20,781	$ 108,747	$ 58,433	$ 336,977
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	7,615	321	7,294
Repurchase of common stock	(383)	(17)	(366)
Increase resulting from stock-based compensation expense	49		49
Other comprehensive income	17,096
Net income	24,911
Dividends paid to public stockholders	(5,107)				(5,107)
Dividends paid to affiliate	(6,710)				(6,710)
Balance at Sep. 30, 2016	562,409	21,085	115,724	75,529	350,071
Balance at Dec. 31, 2016	553,342	21,223	119,054	46,081	366,984
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	5,553	225	5,328
Repurchase of common stock	(1,858)	(68)	(1,790)
Increase resulting from stock-based compensation expense	48		48
Other comprehensive income	18,245
Net income	13,054
Dividends paid to public stockholders	(5,906)				(5,906)
Dividends paid to affiliate	(7,416)				(7,416)
Balance at Sep. 30, 2017	$ 575,062	$ 21,380	$ 122,640	$ 64,326	$ 366,716