Derivative Instruments and Hedging Activities - Additional Information (Detail) In Millions, unless otherwise specified	6 Months Ended											3 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2014 Foreign Currency Derivatives [Member] Cash Flow Hedging [Member] CAD	Jun. 30, 2014 Foreign Currency Derivatives [Member] Cash Flow Hedging [Member] October 31, 2016 [Member] CAD	Jun. 30, 2014 Foreign Currency Derivatives [Member] Cash Flow Hedging [Member] October 31, 2017 [Member] CAD	Jun. 30, 2014 Foreign Currency Derivatives [Member] Cash Flow Hedging [Member] October 31, 2018 [Member] CAD	Jun. 30, 2014 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Dec. 31, 2013 Interest Rate Swaps [Member] Fair Value Hedge [Member] USD ($)	Jun. 30, 2014 Interest Rate Swaps [Member] Fair Value Hedge [Member] Swap One [Member] USD ($)	Jun. 30, 2013 Interest Rate Swaps [Member] Fair Value Hedge [Member] Swap One [Member] USD ($)	Jun. 30, 2014 Interest Rate Swaps [Member] Fair Value Hedge [Member] Swap Two [Member] USD ($)	Jun. 30, 2013 Interest Rate Swaps [Member] Fair Value Hedge [Member] Swap Two [Member] USD ($)	Mar. 31, 2014 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Jun. 30, 2014 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2013 Forward Starting Interest Rate Swap [Member] Cash Flow Hedging [Member] USD ($)	Jun. 30, 2014 Electricity commodity derivatives [Member]	Jun. 30, 2013 Electricity commodity derivatives [Member]	Jun. 30, 2014 Electricity commodity derivatives [Member]	Jun. 30, 2013 Electricity commodity derivatives [Member]	Dec. 31, 2013 Electricity commodity derivatives [Member] MWh	Jun. 30, 2014 Electricity commodity derivatives [Member] Forecast [Member] MWh
Derivative Instruments, Gain (Loss) [Line Items]
Increase in carrying value of debt instruments from fair value hedge accounting for interest rate swaps						$ 51	$ 59
Notional amount of derivatives		370	70	150	150			3	5	8	10	175
Cash received (paid) for termination of interest rate swaps												(36)
Maximum term of cash flow hedges												10 years
Deferred losses, net of taxes, related to cash flow hedges included in accumulated other comprehensive income													53	34
Deferred losses scheduled to be reclassified out of accumulated other comprehensive into interest expense over next 12 months, pre-tax													$ 11
Debt instrument maturity year	2024
Merchant electricity sales hedged or expected to be hedged																			1,730,000	482,000
Percentage of merchant electricity sales hedged															17.00%	56.00%	17.00%	54.00%	56.00%
Expected percentage of merchant electricity sales hedged																				16.00%