CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY AND COMPREHENSIVE INCOME (USD $) In Thousands, except Share data	Common Stock [Member]	Additional Paid In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance at Dec. 31, 2008	$ 93,300	$ 124,708	$ 240,770	$ (742)	$ 458,036
Balance (in shares) at Dec. 31, 2008	933,000
Net income			47,363		47,363
Net derivative instrument changes, net of tax				76	76
Comprehensive income					47,439
Common dividends declared to parent			(34,198)		(34,198)
Contribution of capital by parent		21,797			21,797
Balance at Dec. 31, 2009	93,300	146,505	253,935	(666)	493,074
Balance (in shares) at Dec. 31, 2009	933,000
Net income			42,749		42,749
Net derivative instrument changes, net of tax				76	76
Comprehensive income					42,825
Common dividends declared to parent			(73,787)		(73,787)
Contribution of capital by parent		40,566			40,566
Balance at Dec. 31, 2010	93,300	187,071	222,897	(590)	502,678
Balance (in shares) at Dec. 31, 2010	933,000				933,000
Net income			51,006		51,006
Net derivative instrument changes, net of tax				76	76
Comprehensive income					51,082
Common dividends declared to parent			(32,607)		(32,607)
Contribution of capital by parent		0			0
Balance at Dec. 31, 2011	$ 93,300	$ 187,071	$ 241,296	$ (514)	$ 521,153
Balance (in shares) at Dec. 31, 2011	933,000				933,000