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Debt (Details) (USD $)
In Millions, unless otherwise specified
 11 Months Ended 12 Months Ended 60 Months Ended 84 Months Ended 119 Months Ended 120 Months Ended 360 Months Ended 12 Months Ended 60 Months Ended 12 Months Ended 60 Months Ended
Dec. 09, 2011
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Oct. 27, 2016
Oct. 27, 2018
Jan. 14, 2011
Sep. 15, 2021
Mar. 15, 2041
May 04, 2011
Dec. 31, 2011
Senior notes [Member]
Dec. 31, 2010
Senior notes [Member]
Dec. 31, 2011
Gas facility revenue bonds [Member]
Dec. 31, 2010
Gas facility revenue bonds [Member]
Dec. 31, 2011
Medium-term Notes [Member]
Dec. 31, 2010
Medium-term Notes [Member]
Dec. 31, 2011
AGL Capital interest rate swaps [Member]
Dec. 31, 2010
AGL Capital interest rate swaps [Member]
Dec. 31, 2011
Capital Leases [Member]
Dec. 31, 2010
Capital Leases [Member]
Dec. 31, 2011
First Mortgage [Member]
Dec. 31, 2011
First Mortgage Bond FV Step Up [Member]
Dec. 31, 2011
Debt Instrument Unamortized Discount Premium Net [Member]
Dec. 31, 2010
Debt Instrument Unamortized Discount Premium Net [Member]
Dec. 31, 2011
AGL Capital [Member]
Nov. 10, 2016
AGL Capital [Member]
Dec. 31, 2010
AGL Capital [Member]
Dec. 31, 2011
Nicor Gas [Member]
Dec. 15, 2016
Nicor Gas [Member]
Nov. 02, 2011
SouthStar [Member]
Dec. 31, 2011
Commercial Paper [Member]
AGL Capital [Member]
Dec. 31, 2010
Commercial Paper [Member]
AGL Capital [Member]
Dec. 31, 2011
Commercial Paper [Member]
Nicor Gas [Member]
Dec. 31, 2011
Current portion of long-term debt [Member]
Dec. 31, 2010
Current portion of long-term debt [Member]
Dec. 31, 2011
Noncurrent portion of Capital Lease Obligations [Member]
Dec. 31, 2010
Noncurrent portion of Capital Lease Obligations [Member]
Short-term Debt [Line Items]                                                                          
Short-term Debt, Year(s) due  

2012

                                                                      
Short Term Debt, Amount   $ 1,338 $ 1,033                                                       $ 869 $ 732 $ 452 $ 15 $ 300 $ 2 $ 1
Short-term Debt, Weighted Average Interest Rate   0.70% 3.20%                                                       0.40% 0.40% 0.40% 8.30% 7.10% 4.90% 4.90%
Debt Instrument [Line Items]                                                                          
Long-term Debt, Weighted Average Interest Rate   5.00% 5.20%               5.40% 5.50% 1.20%   7.80% 7.80%       4.90% 5.60%           1.30%                    
Long term debt instruments maturity date range start   2013                                                                      
Long term debt instrument maturity date range end   2041                                                                      
Long-term Debt                     2,550 1,275 200 200 181 196 13 0 0 2 500 99 18 (2)                          
Long-term debt (Note 4 and Note 7)   3,561 1,671                                                                    
Senior Notes   200     120 155 300 300 500                                                        
Senior note maturity Date   Mar. 15, 2041     Oct. 27, 2016 Oct. 27, 2018 Jan. 14, 2011 Sep. 15, 2021 Mar. 15, 2041                                                        
Senior note issuance date   September 15, 2011     October 27, 2011 October 27, 2011   September 15, 2011 March 16, 2011                                                        
Senior Notes Weighted Average Interest Rate   5.90%     1.90% 3.50%   3.50% 5.90% 6.40%                                                      
Payment of senior notes   300 0 0     300                                                            
Line of Credit Facility, Maximum Borrowing Capacity                                                 1,300     700   75              
Line Of Credit Facility Initiation Date                                                 November 10, 2011     December 15, 2011                  
Line Of Credit Facility Expiration Date                                                   November 10, 2016     December 15, 2016 November 2, 2011              
Total debt   4,899 2,704                                                                    
Debt Weighted Average Interest Rate   4.40% 4.60%                                                                    
Debt Details [Abstract]                                                                          
Interest Rate Swap Inception Date   May 04, 2011                                                                      
Derivative Type Of Instrument interest rate swaps interest rate swaps                                                                      
Interest Rate Swap Notional Amount 90 250                                                                      
Interest Rate Swap Maturity Date   Jul. 15, 2016                                                                      
Effective Interest rate of the Interest Rate Swap Description floating rate based on LIBOR

three-month LIBOR plus 3.9%.

                                                                      
Interest Rate Swap Description Of Variable Rate Basis   LIBOR                                                                      
Line of Credit Facility, Description   The credit facilities contain certain non-financial covenants that, among other things, restrict liens and encumbrances, loans and investments, acquisitions, dividends and other restricted payments, asset dispositions, mergers and consolidations and other matters customarily restricted in such agreements.                                                                      
Line of Credit Facility, Covenant Terms   The AGL Credit Facility and the Nicor Gas Credit Facility include a financial covenant that requires us to maintain a ratio of total debt to total capitalization of no more than 70%; however, our goal is to maintain this ratio at levels between 50% and 60%.                                                                      
Line of Credit Facility, Covenant Compliance   We were in compliance with all existing debt provisions and covenants, both financial and non-financial, as of December 31, 2011 and 2010.                                                                      
Adjusted ratio of indebtedness to capital                                                 58.00%   58.00% 60.00%                  
Maturities Of Commercial Paper Range Earliest   Jan. 03, 2012                                                                      
Maturities Of Commercial Paper Range Latest   Feb. 27, 2012                                                                      
Maturities Of Long Term Debt [Abstract]                                                                          
Long Term Debt Maturities Repayment Of Principal In Next Twelve Months   17                                                                      
Long Term Debt Maturities Repayments Of Principal In Year Two   225                                                                      
Long Term Debt Maturities Repayments Of Principal In Year Three   0                                                                      
Long Term Debt Maturities Repayments Of Principal In Year Four   200                                                                      
Long Term Debt Maturities Repayments Of Principa lIn Year Five   545                                                                      
Long Term Debt Maturities Repayments Of Principal After Year Five   $ 2,461