Other Material Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended			3 Months Ended							9 Months Ended
	Aug. 31, 2013	Jun. 30, 2013	Jan. 31, 2013	Mar. 31, 2013	Sep. 30, 2013 Lawsuits	Dec. 31, 2012	Sep. 30, 2013 Con Edison [Member]	Aug. 31, 2013 IRS [Member]	Jun. 30, 2013 IRS [Member]	Sep. 30, 2013 Insurance Receivables [Member]	Sep. 30, 2013 Tax deficiency paid in December 2005 [Member]	Sep. 30, 2013 Interest in April 2006 [Member]	Sep. 30, 2013 Leases [Member]	Sep. 30, 2013 Energy [Member]	Sep. 30, 2013 Solar Energy Projects [Member]	Sep. 30, 2013 Construction [Member]
Operating Leased Assets [Line Items]
Number of suits pending against the company					90
Estimated accrued liability for suits										$ 50
Insurance receivable for suits					50
Total investment in electric generating and gas distribution facilities					259
Transactions financed with equity					93
Transactions financed with non-recourse					166
Tax deficiency paid to IRS											0.3
Interest paid related to tax deficiency												0.2
Disallowed deduction											574
Estimated charge after-tax				150
Gain from LILO transaction termination	26	29
Proceeds from the termination of a LILO transaction	92	108
The company's net investment in LILO transactions						(76)
Gross investment in LILO transactions						228
Deferred tax liabilities						304
Defray of interest charges relating to potential tax liability			447
Returned deposit from IRS					375	70		30	95
Guarantee obligations maximum exposure					1,272	859									80
Percentage of membership interest							50.00%
Guarantees issued													2	25		4
Maximum potential obligation					$ 5