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, 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)
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
Balance at Dec. 31, 2018	565,782	21,615	128,451	1,620	414,096
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	2,151	73	2,078
Repurchase of common stock	(643)	(20)	(623)
Increase resulting from stock-based compensation expense	22		22
Other comprehensive income (loss)	20,470			20,470
Net income (loss)	33,531
Dividends paid to public stockholders	(2,271)				(2,271)
Dividends paid to affiliate	(2,708)				(2,708)
Balance at Mar. 31, 2019	$ 616,334	$ 21,668	$ 129,928	$ 22,090	$ 442,648