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	$ 0	$ 2,886,657	$ 683,516	$ 7,628	$ 3,577,801
Balance (in shares) at Dec. 31, 2008	100
Net income			323,320		323,320
Net derivative instrument changes during the period, net of tax				473	473
Comprehensive income for the period					323,793
Common dividends declared to parent			(264,593)		(264,593)
Contribution of capital by parent		108,813			108,813
Balance at Dec. 31, 2009	0	2,995,470	742,243	8,101	3,745,814
Balance (in shares) at Dec. 31, 2009	100
Net income			399,720		399,720
Net derivative instrument changes during the period, net of tax				(644)	(644)
Comprehensive income for the period					399,076
Common dividends declared to parent			(266,812)		(266,812)
Contribution of capital by parent		260,116			260,116
Balance at Dec. 31, 2010	0	3,255,586	875,151	7,457	4,138,194
Balance (in shares) at Dec. 31, 2010	100				100
Net income			396,803		396,803
Net derivative instrument changes during the period, net of tax				(19,834)	(19,834)
Comprehensive income for the period					376,969
Common dividends declared to parent			(270,245)		(270,245)
Contribution of capital by parent		60,800			60,800
Balance at Dec. 31, 2011	$ 0	$ 3,316,386	$ 1,001,709	$ (12,377)	$ 4,305,718
Balance (in shares) at Dec. 31, 2011	100				100