Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016
Derivative Asset, Fair Value, Gross Asset	$ 3,904		$ 4,514
Derivative Liability, Fair Value, Gross Liability	4,442		4,902
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	21
Netted counterparty balance [Member]
Fair Value, Concentration of Risk, Derivative Instruments, Assets	541		774
Derivative Asset, Fair Value of Collateral	399		339
Derivative Credit Risk Valuation Adjustment, Derivative Assets	5		6
Netted counterparty balance gains [Member]
Fair Value, Concentration of Risk, Derivative Instruments, Assets	940		1,100
Derivative liability positions containing provisions conditioned on downgrades [Member]
Derivative Liability, Fair Value, Gross Liability	1,100		1,100
Additional Termination Event [Member]
Derivative Liability, Fair Value, Gross Liability	1
Additional Termination Event [Member] | Maximum [Member]
Derivative Liability, Fair Value, Gross Liability	17
Credit Support Annex [Member]
Derivative Liability, Fair Value, Gross Liability	1,100
Collateral Already Posted, Aggregate Fair Value	1,000
Credit Support Annex [Member] | Baa3/BBB [Member]
Additional Collateral, Aggregate Fair Value	2
Credit Support Annex [Member] | Ba1/BBB- or below [Member]
Additional Collateral, Aggregate Fair Value	2
Credit Default Swap, Buying Protection [Member]
Derivative, Notional Amount	5		135
Credit Risk Derivatives, at Fair Value, Net			3
Total Return Swap [Member]
Derivative Asset, Fair Value, Gross Asset	15		34
Derivative Liability, Fair Value, Gross Liability	13		31
Collateral Already Posted, Aggregate Fair Value	368		450
Derivative, Notional Amount	$ 1,700		$ 2,100
Financial Guarantee [Member]
Derivative, Average Remaining Maturity	5 years 6 months		8 years 6 months
Credit Derivative, Maximum Exposure, Undiscounted	$ 55		$ 95
Financial Guarantee [Member] | Minimum [Member]
Derivative, Remaining Maturity	1 year		1 year
Financial Guarantee [Member] | Maximum [Member]
Derivative, Remaining Maturity	9 years		31 years
Interest Rate Contract [Member] | Cash Flow Hedging [Member]
Derivative Asset, Fair Value, Gross Asset	$ 2	[1]	$ 34	[2]
Derivative Liability, Fair Value, Gross Liability	$ 252	[1]	$ 265	[2]
Derivative, Average Remaining Maturity	3 years 7 months		4 years 1 month
Interest Rate Contract [Member] | Minimum [Member] | Cash Flow Hedging [Member]
Derivative, Remaining Maturity	1 year		1 year
Interest Rate Contract [Member] | Maximum [Member] | Cash Flow Hedging [Member]
Derivative, Remaining Maturity	5 years		6 years

[1] See “Cash Flow Hedges” in this Note for further discussion.

[2] See “Cash Flow Hedges” in this Note for further discussion.