Derivative Instruments and Hedging Activities - Additional Information (Detail) In Millions, unless otherwise specified								3 Months Ended		12 Months Ended			3 Months Ended
	Mar. 31, 2014 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Dec. 31, 2013 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Mar. 31, 2013 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Mar. 31, 2014 Foreign currency derivatives [Member] Cash Flow Hedging [Member] CAD	Mar. 31, 2014 Foreign currency derivatives [Member] Cash Flow Hedging [Member] October 31, 2016 [Member] CAD	Mar. 31, 2014 Foreign currency derivatives [Member] Cash Flow Hedging [Member] October 31, 2017 [Member] CAD	Mar. 31, 2014 Foreign currency derivatives [Member] Cash Flow Hedging [Member] October 31, 2018 [Member] CAD	Mar. 31, 2014 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2013 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2013 Electricity commodity derivatives [Member] MW	Mar. 31, 2014 Electricity commodity derivatives [Member]	Mar. 31, 2013 Electricity commodity derivatives [Member]	Mar. 31, 2014 Electricity commodity derivatives [Member] Forecast [Member] MW
Derivative Instruments, Gain (Loss) [Line Items]
Increase in carrying value of debt instruments from fair value hedge accounting for interest rate swaps	$ 54	$ 59
Notional amount of derivatives	5		5	370	70	150	150		175
Maximum term of cash flow hedges								10 years
Cash received (paid) for termination of interest rate swaps								(36)
Deferred losses, net of taxes, related to cash flow hedges included in accumulated other comprehensive income								33	34
Deferred losses scheduled to be reclassified out of accumulated other comprehensive into interest expense over next 12 months, pre-tax								$ 7
Merchant electricity sales hedged or expected to be hedged										1,730,000			480,000
Percentage of merchant electricity sales hedged										56.00%			15.00%
Expected percentage of merchant electricity sales hedged											17.00%	53.00%