Fair Value Measurements - Change in Fair Values of Level 3 Financial Instruments (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Securities Available for Sale [Member] \| Other Debt Obligations [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	$ 20	$ 20	$ 20	$ 17
Gains (Losses) Included in Other Comprehensive Income	0	1	0	3
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(1)	0	0
Ending Balance	20	20	20	20
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Other Investments [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	7		3
Gains (Losses) Included in Other Comprehensive Income	0		0
Gains (Losses) Included in Earnings	0		0
Purchases	0		0
Sales	0		0
Settlements	0		0
Transfers Other	0		0
Transfers into Level 3	0		4
Transfers out of Level 3	0		0
Ending Balance	7		7
Unrealized Gains (Losses) Included in Earnings	0		0
Other Investments [Member] \| Principal Investments Direct [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	13	15	13	27
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	(1)	(2)	(1)	(7)
Purchases	0	0	1	0
Sales	0	0	(1)	(7)
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	12	13	12	13
Unrealized Gains (Losses) Included in Earnings	(1)	2	1	(1)
Interest Rate [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	5	16	9	7
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	(2)	0
Purchases	0	0	1	0
Sales	0	0	(2)	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	2	1	6	13
Transfers out of Level 3	(1)	(6)	(6)	(9)
Ending Balance	6	11	6	11
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Credit [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	(9)	1	1	1
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	(6)	(5)	(26)	(12)
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	15	5	25	12
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	0	1	0	1
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Other [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	3	4	3	2
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers Other	(1)	0	(1)	2
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	2	4	2	4
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Loans Held For Sale Fair Value [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	0	0	1	0
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	(1)	(1)	(1)
Settlements	0	0	0	0
Transfers Other	0	3	0	3
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	0	2	0	2
Unrealized Gains (Losses) Included in Earnings	0	$ 0	0	$ 0
Loans Held For Investment Fair Value [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning Balance	3		2
Gains (Losses) Included in Other Comprehensive Income	0		0
Gains (Losses) Included in Earnings	0		0
Purchases	0		0
Sales	0		0
Settlements	0		0
Transfers Other	0		1
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Ending Balance	3		3
Unrealized Gains (Losses) Included in Earnings	$ 0		$ 0

Fair value measurement with unobservable inputs reconciliation recurring basis asset and liabilities value.

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- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Other Transfers

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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