Derivative Instruments and Hedging Activities - Additional Information (Detail) In Millions, unless otherwise specified			1 Months Ended			3 Months Ended	12 Months Ended						1 Months Ended								12 Months Ended		12 Months Ended
	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Apr. 30, 2012 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Dec. 31, 2013 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Dec. 31, 2012 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Sep. 30, 2012 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2013 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2012 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2009 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2012 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] Long-term other liabilities [Member] USD ($)	Dec. 31, 2013 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] Current accrued liabilities [Member] USD ($)	Dec. 31, 2013 Foreign currency derivatives [Member] Principal [Member] October 31, 2016 [Member] CAD	Oct. 31, 2013 Foreign currency derivatives [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2013 Foreign currency derivatives [Member] Cash Flow Hedging [Member] October 31, 2016 [Member] CAD	Dec. 31, 2013 Foreign currency derivatives [Member] Cash Flow Hedging [Member] October 31, 2017 [Member] CAD	Dec. 31, 2013 Foreign currency derivatives [Member] Cash Flow Hedging [Member] October 31, 2018 [Member] CAD	Dec. 31, 2012 Foreign currency derivatives [Member] Cash Flow Hedging [Member] Principal [Member] October 31, 2013 [Member] CAD	Nov. 30, 2012 Foreign currency derivatives [Member] Cash Flow Hedging [Member] Interest [Member] CAD	Nov. 30, 2011 Foreign currency derivatives [Member] Cash Flow Hedging [Member] Interest [Member] CAD	Dec. 31, 2013 Foreign currency derivatives [Member] Cash Flow Hedging [Member] Interest [Member] October 31, 2013 [Member] CAD	Dec. 31, 2013 Treasury rate locks [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2012 Treasury rate locks [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2013 Electricity commodity derivatives [Member] MW	Dec. 31, 2012 Electricity commodity derivatives [Member] MW	Dec. 31, 2011 Electricity commodity derivatives [Member] MW	Dec. 31, 2014 Electricity commodity derivatives [Member] Forecast [Member] MW
Derivative Instruments, Gain (Loss) [Line Items]
Cash received (paid) to settle hedges			$ 76			$ (59)							$ 4
Increase in carrying value of debt instruments from fair value hedge accounting for interest rate swaps				59	79
Notional amount						200			525			370		70	150	150	370	11	10	10
Maximum term of cash flow hedges							10 years														19 years
Deferred losses, net of taxes, related to cash flow hedges included in accumulated other comprehensive income							34	39													6	7
Deferred losses scheduled to be reclassified out of accumulated other comprehensive into interest expense over next 12 months, pre-tax							7														1
Fair value of forward-starting swaps (interest rate derivatives)	$ 28	$ 42								$ 42	$ 28
Merchant electricity sales hedged or expected to be hedged																							1,730,000	630,000	1,550,000	480,000
Percentage of merchant electricity sales hedged																							56.00%	20.00%	50.00%
Expected percentage of merchant electricity sales hedged																										15.00%