Significant Accounting Policies, Regulatory Assets Liability (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Regulatory Assets and Liability Details [Abstract]
Remaining recovery period for postretirement benefit costs	2 to 21
Remaining recovery period for seasonal rates	one year
Recovered pipeline replacement revenue	$ 48	$ 45	$ 41
Non current liability recoverable regulatory infrastructure program costs	145
Current recoverable liability regulatory infrastructure program costs	131
Increase in Accmulated removal cost	1
Nicor Bad Debt Expense	31
Customer Refund Liability	32
Increase In Regulatory Assets	665
Increase In Regulatory Liabilities	1,261
Nicor Gas Regulatory Asset Addition	545
Nicor Gas Regulatory Liability Addition	1,330
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability Current	14	0
Regulatory Liability Non current	1,321	182
Derivative financial instruments [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability Current	22	19
Deferred natural gas costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability Current	53	23
Deferred Income Tax Charges [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability Non current	27	15
Unamortized investment tax credit [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability Non current	32	12
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability Current	15	8
Regulatory Liability Non current	11	16
Total Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,517	256
Regulatory Liability Current	112	31
Regulatory Liability Non current	1,405	225
Bad Debt Rider [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability Current	30	0
Regulatory Liability Non current	14	0
Recoverable regulatory infrastructure program costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets Current	48	48
Regulatory Assets Noncurrent	305	244
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets Current	7	7
Regulatory Assets Noncurrent	351	164
Recoverable seasonal rates [Member]
Regulatory Assets [Line Items]
Regulatory Assets Current	10	11
Postretirement Benefit Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets Current	29	0
Regulatory Assets Noncurrent	262
Asset Recoverable Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets Current		0
Unamortized losses on reacquired debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets Noncurrent	21	11
Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets Current	37
Regulatory Assets Noncurrent	140	34
Total Regulatory Assets [Member]
Regulatory Assets [Line Items]
Regulatory Assets	1,210	545
Regulatory Assets Current	131	92
Regulatory Assets Noncurrent	$ 1,079	$ 453