Derivative Instruments - Additional Information (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivatives designated as cash flow hedges gain (loss), amount expected to be reclassified to future net income (loss), net of tax		$ 1,717	$ 1,863	$ 2,065	$ 2,052	$ 2,064	$ 2,085
Year by which all forecasted transactions associated with qualifying cash flow hedges are expected to occur		2057
Derivatives designated as cash flow hedges gain (loss), amount expected to be reclassified to net income in the next 12 months, net of tax		$ 106
Amount reclassified to net income in connection with forecasted transactions that were no longer considered probable of occurring		6
Derivative assets | Subject to enforceable master netting arrangement
Derivative [Line Items]
Amount to claim from counterparties if the downgrade provisions had been triggered	[1]	$ 79		$ 85

[1] Included $6 million and $2 million of accruals on derivatives classified as other assets and does not include amounts related to embedded derivatives as of September 30, 2018 and December 31, 2017, respectively.