Consolidated Statement of Owner's Equity (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statement of Owner's Equity [Abstract]
Member's Capital balance at beginning of period	$ 1,841,888	$ 1,727,434	$ 1,652,434
Cash contributions from parent	150,000	115,000	75,000
Non-cash contributions from parent	1,524	0	0
Non-cash return of capital	0	(546)	0
Member's Capital balance at end of period	1,993,412	1,841,888	1,727,434
Loans to Parent balance at beginning of period	0	0	(237,526)
Loans to parent, net	0	0	237,526
Loans to Parent balance at end of period	0	0	0
Retained Earnings balance at beginning of period	2,177,811	2,117,153	2,180,367
Net Income	272,466	279,658	270,776
Cash distributions	(246,259)	(219,000)	(333,791)
Non-cash distributions	0	0	(199)
Retained Earnings balance at end of period	2,204,018	2,177,811	2,117,153
Accumulated Other Comprehensive Income (Loss) balance at beginning of period	(346)	173	(718)
Equity interest in unrealized gain (loss) on interest rate hedge	(376)	(519)	891
Accumulated Other Comprehensive Income (Loss) balance at end of period	(722)	(346)	173
Total Owner's Equity	$ 4,196,708	$ 4,019,353	$ 3,844,760