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	4,129	168	3,961
Repurchase of common stock	(1,758)	(64)	(1,694)
Increase resulting from stock-based compensation expense	30		30
Other comprehensive income (loss)	13,299			13,299
Net income (loss)	12,308				12,308
Dividends paid to public stockholders	(3,924)				(3,924)
Dividends paid to affiliate	(4,944)				(4,944)
Balance at Jun. 30, 2017	572,482	21,327	121,351	59,380	370,424
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	3,239	127	3,112
Repurchase of common stock	(1,455)	(56)	(1,399)
Increase resulting from stock-based compensation expense	39		39
Other comprehensive income (loss)	(23,094)			(23,094)
Net income (loss)	(5,071)				(5,071)
Dividends paid to public stockholders	(4,257)				(4,257)
Dividends paid to affiliate	(5,180)				(5,180)
Balance at Jun. 30, 2018	568,067	$ 21,526	$ 126,308	(5,944)	426,177
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	$ 0			$ (66,234)	$ 66,234