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, 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 (loss)	18,245			18,245
Net income	13,054				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
Balance at Dec. 31, 2017	603,846	21,455	124,556	83,384	374,451
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	4,478	176	4,302
Repurchase of common stock	(1,455)	(56)	(1,399)
Increase resulting from stock-based compensation expense	61		61
Other comprehensive income (loss)	(32,232)			(32,232)
Net income	14,077				14,077
Dividends paid to public stockholders	(6,398)				(6,398)
Dividends paid to affiliate	(7,769)				(7,769)
Balance at Sep. 30, 2018	574,608	$ 21,575	$ 127,520	(15,082)	440,595
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	$ 0			$ (66,234)	$ 66,234