Fair Value Measurements - Additional Information Relating To Asset-Backed Securities and Collateralized Debt Obligations (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Level 2 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 233
Level 2 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	248
Level 3 [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	269
Level 3 [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Estimate of Fair Value Measurement [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	502
Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	248
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	228
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	247
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
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
AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	228
AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	247
Commercial Mortgages [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	60
Commercial Mortgages [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
Commercial Mortgages [Member] | AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	60
Home Equity [Member] | AAA - A Rating [Member] | Level 2 [Member] | Alt [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	92
Home Equity [Member] | AAA - A Rating [Member] | Level 3 [Member] | Alt [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Home Equity [Member] | AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Alt [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	92
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	95
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	0
Other [Member] | AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	95
Residential Mortgages [Member] | AAA - A Rating [Member] | Level 2 [Member] | Alt [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	87
Residential Mortgages [Member] | AAA - A Rating [Member] | Level 2 [Member] | Subprime [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	57
Residential Mortgages [Member] | AAA - A Rating [Member] | Level 3 [Member] | Alt [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Residential Mortgages [Member] | AAA - A Rating [Member] | Level 3 [Member] | Subprime [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Residential Mortgages [Member] | AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Alt [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	87
Residential Mortgages [Member] | AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Subprime [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	57
Residential Mortgages [Member] | CCC - Unrated [Member] | Level 2 [Member] | Alt [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	1
Residential Mortgages [Member] | CCC - Unrated [Member] | Level 2 [Member] | Subprime [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	5
Residential Mortgages [Member] | CCC - Unrated [Member] | Level 3 [Member] | Alt [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Residential Mortgages [Member] | CCC - Unrated [Member] | Level 3 [Member] | Subprime [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	0
Residential Mortgages [Member] | CCC - Unrated [Member] | Estimate of Fair Value Measurement [Member] | Alt [Member] | Available-for-sale Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	1
Residential Mortgages [Member] | CCC - Unrated [Member] | Estimate of Fair Value Measurement [Member] | Subprime [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	5
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	269
Collateralized debt obligations [Member] | BBB -B Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	269
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	84
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
Student loans [Member] | AAA - A Rating [Member] | Estimate of Fair Value Measurement [Member] | Trading Securities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Trading asset-backed securities and related collateral	$ 84