Condensed Consolidated Statement of Financial Position (Unaudited) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Current assets
Cash and cash equivalents	$ 149		$ 131		$ 71
Short-term investments	43		58		57
Receivables
Gas, unbilled and other receivables	874		686		577
Energy marketing receivables	627		677		386
Less allowance for uncollectible accounts	39		28		19
Total receivables	1,462		1,335		944
Inventories, net	393		708		464
Regulatory assets	119		145		137
Derivative instruments	100		130		218
Other current assets	95		161		131
Total current assets	2,361		2,668		2,022
Long-term assets and other deferred debits
Property, plant and equipment	10,610		10,478		9,920
Less accumulated depreciation	2,202		2,131		1,947
Property, plant and equipment, net	8,408		8,347		7,973
Goodwill	1,883		1,837		1,813
Regulatory assets	878		944		1,057
Intangible assets	156		96		102
Derivative instruments	11		14		48
Other	243		235		224
Total long-term assets and other deferred debits	11,579		11,473		11,217
Total assets	13,940	[1]	14,141	[2]	13,239	[3]
Current liabilities
Short-term debt	868		1,377		730
Energy marketing trade payable	653		611		425
Accounts payable - trade	314		334		205
Current portion of long-term debt and capital leases	226		226		17
Regulatory liabilities	238		161		173
Accrued expenses	166		140		145
Temporary LIFO liquidation	179				89
Customer deposit and credit balances	115		143		130
Accrued regulatory infrastructure program costs	83		121		149
Accrued environmental remediation liabilities	63		57		39
Derivative instruments	20		33		93
Other current liabilities	135		135		153
Total current liabilities	3,060		3,338		2,348
Long-term liabilities and other deferred credits
Long-term debt	3,324		3,327		3,558
Accumulated deferred income taxes	1,568		1,588		1,447
Regulatory liabilities	1,498		1,477		1,431
Accrued environmental remediation liabilities	362		387		289
Accrued retiree welfare benefits	266		268		318
Accrued pension obligations	243		240		228
Derivative instruments	4		6		10
Other long-term liabilities and other deferred credits	74		75		184
Total long-term liabilities and other deferred credits	7,339		7,368		7,465
Total liabilities and other deferred credits	10,399		10,706		9,813
Equity
Common stock, $5 par value; 750,000,000 shares authorized: outstanding: 118,123,770 shares at March 31, 2013, 117,855,075 shares at December 31, 2012 and 117,282,844 shares at March 31, 2012	592		590		587
Additional paid in capital	2,019		2,014		1,994
Retained earnings	1,134		1,035		1,055
Accumulated other comprehensive loss	(211)	[4]	(218)	[4]	(218)
Treasury shares, at cost: 216,523 shares at March 31, 2013 and December 31, 2012 and 213,341 shares at March 31, 2012	(8)		(8)		(8)
Total common shareholders’ equity	3,526		3,413		3,410
Noncontrolling interest	15		22		16
Total equity	3,541		3,435		3,426
Total liabilities and equity	$ 13,940		$ 14,141		$ 13,239

[1] Transaction expenses associated with the Nicor merger are shown separately to better compare year-over-year results.

[2] Identifiable assets are those assets used in each segment's operations.

[3] Identifiable assets are those used in each segment's operations.

[4] All amounts are net of income taxes. Amounts in parentheses indicate debits to Accumulated Other Comprehensive Loss.