Other Material Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended					9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Leases [Member]	Sep. 30, 2012 Energy [Member]	Sep. 30, 2012 Construction [Member]	Sep. 30, 2012 Tax Deficiency Paid In December 2005 [Member]	Sep. 30, 2012 Interest In April 2006 [Member]	Sep. 30, 2012 Disallowance Of Tax 1998-2007 [Member]	Dec. 31, 2011 Disallowance Of Tax 2011 [Member]	Dec. 31, 2010 Disallowance Of Tax 2010 [Member]	Dec. 31, 2009 Disallowance Of Tax 2009 [Member]	Dec. 31, 2008 Disallowance Of Tax 2008 [Member]
Operating Leased Assets [Line Items]
Total investment in electric generating and gas distribution facilities	$ 259
Transactions financed with equity	93
Transactions financed with non-recourse	166
The company's investment in leveraged leases	(73)	(55)
Gross investment in leverage leases	228	234
Deferred tax liabilities	301	289
Tax deficiency paid to IRS						0.3
Interest paid related to tax deficiency							0.2
Disallowed deduction								416	35	40	41	42
Tax savings from lease in lease out transactions	248
Interest required to be paid	122
Guarantee obligations maximum exposure	823	760
Guarantees issued			7	25	11
Maximum potential obligation	$ 5