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	1,953	76	1,877
Repurchase of common stock	(1,758)	(64)	(1,694)
Increase resulting from stock-based compensation expense	7		7
Other comprehensive income (loss)	8,354			8,354
Net income (loss)	6,804				6,804
Dividends paid to public stockholders	(1,956)				(1,956)
Dividends paid to affiliate	(2,472)				(2,472)
Balance at Mar. 31, 2017	564,274	21,235	119,244	54,435	369,360
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	2,398	94	2,304
Repurchase of common stock	(802)	(30)	(772)
Increase resulting from stock-based compensation expense	18		18
Other comprehensive income (loss)	(19,317)			(19,317)
Net income (loss)	(76)				(76)
Dividends paid to public stockholders	(2,128)				(2,128)
Dividends paid to affiliate	(2,590)				(2,590)
Balance at Mar. 31, 2018	581,349	$ 21,519	$ 126,106	(2,167)	435,891
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative adjustment for adoption of financial instruments recognition and measurement changes	$ 0			$ (66,234)	$ 66,234