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Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail)
$ in Millions
Mar. 31, 2018
USD ($)
[1]
Level 2 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral $ 109
Level 3 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 131
AAA - A Rating [Member] | Level 2 [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 249
AAA - A Rating [Member] | Level 3 [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 107
Student loans [Member] | AAA - A Rating [Member] | Level 2 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 92
Student loans [Member] | AAA - A Rating [Member] | Level 3 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 0
Collateralized debt obligations [Member] | BBB -B Rating [Member] | Level 2 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 0
Collateralized debt obligations [Member] | BBB -B Rating [Member] | Level 3 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 131
Other [Member] | AAA - A Rating [Member] | Level 2 [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 200
Other [Member] | AAA - A Rating [Member] | Level 3 [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 107
Estimate of fair value [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 240
Estimate of fair value [Member] | AAA - A Rating [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 356
Estimate of fair value [Member] | Student loans [Member] | AAA - A Rating [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 92
Estimate of fair value [Member] | Collateralized debt obligations [Member] | BBB -B Rating [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 131
Estimate of fair value [Member] | Other [Member] | AAA - A Rating [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 307
Alt [Member] | Home Equity [Member] | AAA - A Rating [Member] | Level 2 [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 49
Alt [Member] | Home Equity [Member] | AAA - A Rating [Member] | Level 3 [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 0
Alt [Member] | Estimate of fair value [Member] | Home Equity [Member] | AAA - A Rating [Member] | Available-for-sale Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 49
Subprime [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Level 2 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 17
Subprime [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Level 3 [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral 0
Subprime [Member] | Estimate of fair value [Member] | Residential Mortgages [Member] | CCC - Unrated [Member] | Trading Securities [Member]  
Fair Value Of Other Financial Instrument [Line Items]  
Trading asset-backed securities and related collateral $ 17
[1] We utilize Standard and Poor's ("S&P") as the primary source of credit ratings in the tables above. If S&P ratings are not available, ratings by Moody's and Fitch are used in that order. Ratings for collateralized debt obligations represent the ratings associated with the underlying collateral.