Consolidated Statement of Owner's Equity (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Consolidated Statement Of Owner's Equity [Abstract]
Member's Capital balance at beginning of period	$ 1,727,434	$ 1,652,434	$ 1,652,430
Cash contributions from parent	115,000	75,000	0
Non-cash contributions from parent	0	0	4
Non-cash return of capital	(546)	0	0
Member's Capital balance at end of period	1,841,888	1,727,434	1,652,434
Loans to Parent balance at beginning of period	0	(237,526)	(42,206)
Loans to parent, net	0	237,526	(195,320)
Loans to Parent balance at end of period	0	0	(237,526)
Retained Earnings balance at beginning of period	2,117,153	2,180,367	2,045,010
Net Income	279,658	270,776	280,357
Cash distributions	(219,000)	(333,791)	(145,000)
Non-cash distributions	0	(199)	0
Retained Earnings balance at end of period	2,177,811	2,117,153	2,180,367
Accumulated Other Comprehensive Income (Loss) balance at beginning of period	173	(718)	(1,087)
Equity interest in unrealized gain (loss) on interest rate hedge	(519)	891	369
Accumulated Other Comprehensive Income (Loss) balance at end of period	(346)	173	(718)
Total Owner's Equity	$ 4,019,353	$ 3,844,760	$ 3,594,557