CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - 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, 2015	$ 524,938	$ 20,781	$ 108,747	$ 58,433	$ 336,977
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	11,070	459	10,611
Repurchase of common stock	(383)	(17)	(366)
Increase resulting from stock-based compensation expense	62		62
Other comprehensive income (loss)	(12,352)			(12,352)
Net income (loss)	46,203				46,203
Dividends paid to public stockholders	(7,014)				(7,014)
Dividends paid to affiliate	(9,182)				(9,182)
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	7,527	300	7,227
Repurchase of common stock	(1,858)	(68)	(1,790)
Increase resulting from stock-based compensation expense	65		65
Other comprehensive income (loss)	23,530			23,530
Net income (loss)	39,238				39,238
Dividends paid to public stockholders	(7,992)				(7,992)
Dividends paid to affiliate	(10,006)				(10,006)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA	0			13,773	(13,773)
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	5,426	216	5,210
Repurchase of common stock	(1,455)	(56)	(1,399)
Increase resulting from stock-based compensation expense	84		84
Other comprehensive income (loss)	(15,530)			(15,530)
Net income (loss)	(7,468)				(7,468)
Dividends paid to public stockholders	(8,644)				(8,644)
Dividends paid to affiliate	(10,477)				(10,477)
Balance at Dec. 31, 2018	565,782	$ 21,615	$ 128,451	$ 1,620	414,096
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 0				$ 66,234