Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended			120 Months Ended	360 Months Ended
	Mar. 31, 2011	Sep. 30, 2011	Sep. 30, 2010		Jan. 14, 2011	Mar. 15, 2041	Sep. 15, 2021	Oct. 27, 2018	Oct. 27, 2016	Oct. 27, 2011	Sep. 15, 2011	Dec. 31, 2010
Short-term Debt [Line Items]
Short-term Debt, Year(s) due		2011-2012
Short-term debt amount outstanding		$ 17	$ 975									$ 1,033
Short-term Debt, Weighted Average Interest Rate		0.80%	3.50%	[1]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		5.10%	5.40%	[2]
Long term debt instruments maturity date range start		2013
Long term debt instrument maturity date range end		2041
Long-term Debt		11	0									0
Long-term debt (Note 4 and Note 7)		2,687	1,512									1,671
Total debt		2,704	2,487									2,704
Debt Weighted Average Interest Rate		4.50%	4.90%
Debt Details [Abstract]
Senior note issuance date	March 16, 2011
Senior Notes	500	300	300	[3]		200	300	155	120	275	500	300	[3]
Senior Notes Weighted Average Interest Rate	5.90%	6.40%				5.90%	3.50%	3.50%	1.90%
Senior note maturity Date					Jan. 14, 2011	Mar. 15, 2041
Payment of senior notes		300	0
Interest Rate Swap Inception Date		May 04, 2011
Derivative Type Of Instrument		interest rate swaps
Interest Rate Swap Notional Amount		250
Interest Rate Swap Maturity Date		Jul. 15, 2016
Effective Interest rate of the Interest Rate Swap Description		the three-month London Inter-bank Offered Rate (LIBOR) plus 3.9%
Interest Rate Swap Description Of Variable Rate Basis		London Inter-bank Offered Rate (LIBOR)
Line of Credit Facility, Description		The Credit Facility contains certain non-financial covenants that, among other things, restrict liens and encumbrances, loans and investments, restricted payments, asset dispositions, fundamental changes and other matters customarily restricted in such agreements.
Line of Credit Facility, Covenant Terms		Our Credit Facility includes a financial covenant that requires us to maintain a ratio, on a consolidated basis, of total debt to total capitalization of no more than 70%; however, our goal is to maintain this ratio at a level between 50% and 60%.
Line of Credit Facility, Covenant Compliance		We are currently in compliance with all existing debt provisions and covenants. Our Bridge Facility contains the same financial covenant and similar non-financial covenants and default provisions; however, most of these are not in effect until we draw under the facility.
Adjusted ratio of indebtedness to capital		57.00%	56.00%									58.00%
Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt amount outstanding		0	674									732
Short-term Debt, Weighted Average Interest Rate		0.40%	0.40%
Senior notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		5.50%	5.50%
Long-term Debt		2,275	1,275									1,275
Current portion of long-term debt [Member]
Short-term Debt [Line Items]
Short-term debt amount outstanding		15	300									300
Short-term Debt, Weighted Average Interest Rate		8.30%	7.10%
Capital Leases [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		0.00%	4.90%
Long-term Debt		0	3									2
Noncurrent portion of Capital Lease Obligations [Member]
Short-term Debt [Line Items]
Short-term debt amount outstanding		2	1									1
Short-term Debt, Weighted Average Interest Rate		4.90%	4.90%
Gas facility revenue bonds [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		1.10%	5.30%
Long-term Debt		200	40									200
Medium-term Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		7.80%	7.80%
Long-term Debt		181	196									196
AGL Capital interest rate swaps [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		4.20%	0.00%
Debt Instrument Unamortized Discount Premium Net [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate			0.00%
Long-term Debt		$ 20	$ (2)									$ (2)

[1] Excluding the $300 million of senior notes repaid in January 2011, the weighted-average short-term interest rate for the nine months ended September 30, 2010 was 0.4%.

[2] Including the $300 million of senior notes repaid in January 2011, the weighted-average long-term interest rate for the nine months ended September 30, 2010 was 5.6%.

[3] September 30, 2011 includes $15 million of medium-term notes that are classified in current portion of long-term debt and are due in 2012. December 31, 2010 includes $300 million and September 30, 2010 includes $300 million of senior notes repaid in January 2011.