Variable Interest Entities and Securitizations - Additional Information (Detail) (USD $)	3 Months Ended
	Dec. 31, 2013 trust	Dec. 31, 2014
Variable Interest Entity [Line Items]
Funded HELOCs advances	$ 29,000,000	$ 30,000,000
Unfunded commitment on residual interests on trusts	7,000,000	6,000,000
Maximum exposure to loss	3,767,000,000	4,451,000,000
Carrying amounts of assets, unconsolidated	3,264,000,000	3,796,000,000
VIEs consolidated assets	40,812,000,000	37,157,000,000
Other VIE assets held in trust	204,000,000	159,000,000
Affordable housing entities
Variable Interest Entity [Line Items]
Maximum exposure to loss	2,969,000,000	3,500,000,000
Carrying amounts of assets, unconsolidated	2,969,000,000	3,500,000,000
Total assets of the unconsolidated VIE investment funds	9,800,000,000	10,200,000,000
VIEs consolidated assets	0	0
Entities that provide capital to low-income and rural communities
Variable Interest Entity [Line Items]
Maximum exposure to loss	1,000,000	1,000,000
Carrying amounts of assets, unconsolidated	1,000,000	1,000,000
VIEs consolidated assets	389,000,000	374,000,000
Other
Variable Interest Entity [Line Items]
Maximum exposure to loss	95,000,000	74,000,000
Carrying amounts of assets, unconsolidated	95,000,000	74,000,000
VIEs consolidated assets	1,000,000	4,000,000
Retained interest in the trust	93,000,000	74,000,000
Maximum loss exposure to royalty payment	93,000,000	74,000,000
GreenPoint HELOCs
Variable Interest Entity [Line Items]
Number of deconsolidated VIE trust in which entity is primary beneficiary	1
Removal of asset as a result of deconsolidation	33,000,000
Removal of liabilities as a result of deconsolidation	30,000,000
Removal of charge from income statement as a result of deconsolidation	3,000,000
Variable Interest Entity, Not Primary Beneficiary | Option ARM
Variable Interest Entity [Line Items]
Securities held by third-party investors, mortgage	2,320,000,000	2,026,000,000
Unpaid principal balance of securitization trusts	2,399,000,000	2,094,000,000
Variable Interest Entity, Not Primary Beneficiary | GreenPoint HELOCs
Variable Interest Entity [Line Items]
Securities held by third-party investors, mortgage	122,000,000	95,000,000
Unpaid principal balance of securitization trusts	116,000,000	89,000,000
Variable Interest Entity, Not Primary Beneficiary | GreenPoint Manufactured Housing
Variable Interest Entity [Line Items]
Securities held by third-party investors, mortgage	994,000,000	887,000,000
Unpaid principal balance of securitization trusts	1,000,000,000	893,000,000
Maximum exposure to loss		$ 420,000,000