CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash flow from operating activities
Net (loss) income	$ 186	$ 250	$ 249
Change in derivative financial instrument assets and liabilities	(24)	(2)	11
Depreciation and amortization	186	160	158
Deferred income taxes	213	92	105
Inventories	158	33	(9)
Energy marketing receivables and energy marketing trade payables, net (Note 2)	27	47	(81)
Deferred natural gas costs	(3)	(14)	24
Gas, unbilled and other receivables net of allowance	45	(26)	108
Accrued expenses	(77)	7	19
Gas and trade payables	(70)	(12)	1
Increase Decrease In Prepaid Expense	98	6	21
Other - net	(92)	(15)	28
Net cash flow provided by operating activities	451	526	592
Cash flows from investing activities
Payments to acquire property, plant and equipment	(427)	(510)	(476)
Proceeds from disposition of assets	0	73	0
Cash consideration paid in Nicor Merger, net of cash received	(912)	0	0
Other	0	(5)	0
Net cash flow used in investing activities	(1,339)	(442)	(476)
Cash flows from financing activities
Net payments and borrowings of short-term debt	91	131	(264)
Payment of senior notes	(300)	0	0
Dividends paid on common shares	(148)	(133)	(127)
Distribution to noncontrolling interest (Note 8)	(16)	(27)	(20)
Purchase of treasury shares	(2)	(7)	0
Issuance of senior notes	1,014	0	297
Purchase of additional 15% ownership interest in SouthStar	0	(58)	0
Payments of gas facility revenue bonds	0	(160)	0
Benefit dividend reinvestment and stock purchase plans	19	8	8
Issuance of variable rate gas facility revenue bonds	0	160	0
Proceeds From Issuance Of Debt	275	0	0
Proceeds From Lines Of Credit	150	0	0
Repayments Of Lines Of Credit	(150)	0	0
Net cash flow used in financing activities	933	(86)	(106)
Net increase (decrease) in cash and cash equivalents	45	(2)	10
Cash and cash equivalents at beginning of period	24	26	16
Cash and cash equivalents at end of period	69	24	26
Cash paid during the period for
Interest	116	107	93
Income taxes	12	58	50
Cash Flow Noncash Investing And Financing Activities Disclosure [Abstract]
Noncash Or Part Noncash Acquisition Noncash Financial Or Equity Instrument Consideration Value Of Shares Issued	$ 1,523	$ 0	$ 0