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, 2014	$ 502,886	$ 20,344	$ 99,891	$ 81,662	$ 300,989
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through stock plans	9,078	437	8,641
Increase resulting from stock-based compensation expense	215		215
Other comprehensive income (loss)	(23,229)			(23,229)
Net income	50,162				50,162
Dividends paid to public stockholders	(6,012)				(6,012)
Dividends paid to affiliate	(8,162)				(8,162)
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	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	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