CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (USD $) In Thousands, unless otherwise specified	Total		Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]
Balance at Dec. 31, 2011	$ 352,341		$ 12,876	$ 88,311	$ 27,340	$ 223,814
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through affiliate's stock plans	692		34	658
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	237			237
Other comprehensive income	20,412				20,412
Net income	37,966					37,966
Dividends paid to public stockholders	(4,082)					(4,082)
Dividends paid to affiliate	(6,357)					(6,357)
Balance at Dec. 31, 2012	401,209		12,910	89,206	47,752	251,341
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through affiliate's stock plans	10,208		396	9,812
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	291			291
Other comprehensive income	11,258	[1]			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		13,306	99,309	59,010	283,585
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through affiliate's stock plans	7,392		257	7,135
Increase resulting from stock-based compensation expense associated with affiliate's stock plans allocated to the Company	228			228
Other comprehensive income	22,652	[1]			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		$ 13,563	$ 106,672	$ 81,662	$ 300,989

[1] All amounts are net of tax. Amounts in parentheses indicate debits.