Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			119 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jan. 14, 2011	Mar. 15, 2041	Sep. 15, 2021	Oct. 27, 2018	Oct. 27, 2016	Dec. 09, 2011
Notes To Financial Statements [Abstract]
Long-term debt carrying amount (1)	$ 3,576	$ 1,971
Long-term debt fair value	3,938	2,122
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	299	228
Liabilities	(98)	(48)
Footnotes To Fair Value, Assets and Liabilities measured on recurring basis [Abstract]
Weather Derivative Premium	3	1
Footnotes To Carrying and Fair Value of Debt [Abstract]
Medium Term Notes	15
Senior Notes	200			300	500	300	155	120
Payment of senior notes	300	0	0	300
Business Acquisition Purchase Price Allocation Noncurrent Liabilities Long Term Debt	500								(599)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	38	22
Liabilities	(145)	(71)
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	229	153
Liabilities	(68)	(29)
Fair Value, Inputs, Level 2 [Member] | Interest rate swap contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	13	0
Liabilities	(13)	0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Liabilities	0	0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	53
Fair Value, Measurements, Recurring [Member] | Netting And Collateral [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	32	53
Liabilities	$ 115	$ 52