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, 2012	$ 401,209	$ 19,364	$ 82,752	$ 47,752	$ 251,341
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through affiliate's stock plans	10,208	595	9,613
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	291		291
Other comprehensive income (loss)	11,258			11,258
Net income	43,519				43,519
Dividends paid to public stockholders	(4,526)				(4,526)
Dividends paid to affiliate	(6,749)				(6,749)
Balance at Dec. 31, 2013	455,210	19,959	92,656	59,010	283,585
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through affiliate's stock plans	7,392	385	7,007
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	228		228
Other comprehensive income (loss)	22,652			22,652
Net income	29,992				29,992
Dividends paid to public stockholders	(5,211)				(5,211)
Dividends paid to affiliate	(7,377)				(7,377)
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 affiliate's stock plans	9,078	437	8,641
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	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