Derivative Instruments and Hedging Activities - Additional Information (Detail)	1 Months Ended	3 Months Ended			9 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended							3 Months Ended		9 Months Ended		12 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Apr. 30, 2012 USD ($)	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Mar. 31, 2011 USD ($)	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Dec. 31, 2011 USD ($)	Sep. 30, 2012 Forward Starting Interest Rate Swap [Member] USD ($)	Sep. 30, 2011 Forward Starting Interest Rate Swap [Member] USD ($)	Sep. 30, 2012 Forward Starting Interest Rate Swap [Member] USD ($)	Sep. 30, 2011 Forward Starting Interest Rate Swap [Member] USD ($)	Dec. 31, 2009 Forward Starting Interest Rate Swap [Member] USD ($)	Mar. 31, 2011 Forward Starting Interest Rate Swap [Member] USD ($)	Sep. 30, 2012 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2011 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Sep. 30, 2012 Principal [Member] October 31, 2013 [Member] CAD	Sep. 30, 2012 Interest [Member] October 31, 2013 [Member] CAD	Sep. 30, 2012 Interest [Member] November 30, 2012 [Member] CAD	Sep. 30, 2012 Electricity commodity contracts [Member] USD ($)	Sep. 30, 2011 Electricity commodity contracts [Member]	Sep. 30, 2012 Electricity commodity contracts [Member] USD ($) MW	Sep. 30, 2011 Electricity commodity contracts [Member]	Dec. 31, 2011 Electricity commodity contracts [Member] MW	Sep. 30, 2012 Electricity commodity contracts [Member] Maximum [Member] USD ($)	Sep. 30, 2012 Foreign exchange contracts [Member] USD ($)	Sep. 30, 2011 Foreign exchange contracts [Member] USD ($)	Sep. 30, 2012 Foreign exchange contracts [Member] USD ($)	Sep. 30, 2011 Foreign exchange contracts [Member] USD ($)	Sep. 30, 2012 Treasury Lock [Member] USD ($)	Sep. 30, 2011 Treasury Lock [Member] USD ($)	Sep. 30, 2012 Treasury Lock [Member] USD ($)	Sep. 30, 2011 Treasury Lock [Member] USD ($)	Dec. 31, 2011 Treasury Lock [Member] USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Fixed-rate senior notes outstanding		$ 6,600,000,000			$ 6,600,000,000		$ 6,100,000,000
Senior notes swapped to variable interest rates	1,000,000,000						1,000,000,000
Percentage of senior notes swapped to variable interest rates							16.00%
Proceeds from termination of interest rate swaps	76,000,000
Increase in carrying value of debt instruments from fair value hedge accounting for interest rate swaps		85,000,000			85,000,000		102,000,000
Notional amount of forward-starting interest rate swaps								200,000,000		200,000,000		525,000,000	150,000,000
Maximum term of cash flow hedges												10 years
Cash paid to settle forward-starting interest rate swap agreements		59,000,000		9,000,000
Tenor of senior notes issued in February 2011 in years				10 years
Fair value of forward-starting swaps (interest rate derivatives) included in long-term liabilities														42,000,000	32,000,000
Pretax gains (losses) recognized in other comprehensive income due to changes in fair value of cash flow hedges								(3,000,000)	(46,000,000)	(28,000,000)	(53,000,000)
After tax gains (losses) recognized in other comprehensive income due to changes in fair value of cash flow hedges		(13,000,000)	(12,000,000)		(25,000,000)	(25,000,000)		(2,000,000)	(28,000,000)	(17,000,000)	(33,000,000)								(2,000,000)					1,000,000	(9,000,000)	15,000,000	(8,000,000)	7,000,000
Deferred losses reclassified out of accumulated other comprehensive income into interest expense, pre tax								2,000,000		1,000,000																			2,000,000	2,000,000	6,000,000	6,000,000
Deferred gains (losses) related to cash flow hedges reclassified out of accumulated other comprehensive income into income, after tax		10,000,000	(18,000,000)		10,000,000	(9,000,000)		2,000,000		1,000,000									1,000,000		5,000,000				(8,000,000)	20,000,000	(10,000,000)	13,000,000	2,000,000	1,000,000	4,000,000	3,000,000
Deferred losses scheduled to be reclassified out of accumulated other comprehensive into interest expense over next 12 months, pre-tax								7,000,000		7,000,000																			2,000,000		2,000,000
Deferred losses, net of taxes, related to Treasury rate locks included in accumulated other comprehensive income																													8,000,000		8,000,000		12,000,000
Maximum term of cash flow hedges																															2032
Notional amount of foreign currency cash flow hedges																370,000,000	10,000,000	11,000,000
Percentage of merchant electricity sales hedged or expected to be hedged																					628,800.00		1,550,000
Percentage of merchant electricity sales hedged																			16.00%	46.00%	21.00%	49.00%	50.00%
Expected percentage of merchant electricity sales hedged																			20.00%		20.00%
Pre tax gains (losses) recognized in other comprehensive income for changes in fair value of electricity commodity derivatives																			(3,000,000)		1,000,000
Pre-tax gains (losses) associated with electricity commodity derivatives reclassified from accumulated other comprehensive income into income																			$ 2,000,000		$ 9,000,000