Derivative Instruments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) Bcf	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Bcf
Derivative [Line Items]
Cash payment for interest-rate swap agreements	$ 37,000,000	$ 555,000,000
Debt instrument, principal amount	16,698,000,000		$ 16,706,000,000
Amount of collateral posted related to derivative instruments with credit-risk-related contingent features	130,000,000		117,000,000
Total Amount of Senior Notes Due in 2019 [Member]
Derivative [Line Items]
Debt instrument, principal amount	$ 900,000,000
Zero-Coupon Senior Notes [Member]
Derivative [Line Items]
Debt instrument, maturity date	Oct. 10, 2036
Debt Instrument, earliest call date	Oct. 10, 2017
Zero-Coupon Senior Notes [Member] | Accreted Value at Next Potential Put Date [Member]
Derivative [Line Items]
Debt instrument, accreted value	$ 883,000,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Gross derivative liabilities	1,406,000,000		1,592,000,000
Aggregate fair value of derivative instruments with credit-risk-related contingent features for which a net liability position existed (net of collateral)	1,200,000,000		$ 1,400,000,000
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Cash payment for interest-rate swap agreements	24,000,000	$ 193,000,000
Not Designated as Hedging Instrument [Member] | Interest-Rate Swaps 1 Through 5 [Member]
Derivative [Line Items]
Notional principal amount of interest-rate swap	$ 1,600,000,000
Natural Gas [Member] | Commodity [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Financial derivative transactions | Bcf	2		2