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	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
Balance at Mar. 31, 2018	581,349	21,519	126,106	(2,167)	435,891
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	841	33	808
Repurchase of common stock	(653)	(26)	(627)
Increase resulting from stock-based compensation expense	21		21
Other comprehensive income (loss)	(3,777)			(3,777)
Net income (loss)	(4,995)				(4,995)
Dividends paid to public stockholders	(2,129)				(2,129)
Dividends paid to affiliate	(2,590)				(2,590)
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
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	2,191	77	2,114
Repurchase of common stock	(643)	(20)	(623)
Increase resulting from stock-based compensation expense	19		19
Other comprehensive income (loss)	38,356			38,356
Net income (loss)	34,816				34,816
Dividends paid to public stockholders	(4,544)				(4,544)
Dividends paid to affiliate	(5,415)				(5,415)
Balance at Jun. 30, 2019	630,562	21,672	129,961	39,976	438,953
Balance at Mar. 31, 2019	616,334	21,668	129,928	22,090	442,648
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	40	4	36
Increase resulting from stock-based compensation expense	(3)		(3)
Other comprehensive income (loss)	17,886			17,886
Net income (loss)	1,285				1,285
Dividends paid to public stockholders	(2,273)				(2,273)
Dividends paid to affiliate	(2,707)				(2,707)
Balance at Jun. 30, 2019	$ 630,562	$ 21,672	$ 129,961	$ 39,976	$ 438,953