Loans and asset quality- Information about Past Due Loans (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 176		$ 156
Domestic
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	126		156
Domestic | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6		0
Domestic | Wealth management loans and mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	48		46
Domestic | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	43		79
Domestic | Other residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	29		31
Foreign | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	50	[1]	0	[1]
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	147		147
30 to 59 Days Past Due | Domestic
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	117		147
30 to 59 Days Past Due | Domestic | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	6		0
30 to 59 Days Past Due | Domestic | Wealth management loans and mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	48		45
30 to 59 Days Past Due | Domestic | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	43		79
30 to 59 Days Past Due | Domestic | Other residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	20		23
30 to 59 Days Past Due | Foreign | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	30	[1]	0	[1]
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	25		3
60 to 89 Days Past Due | Domestic
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5		3
60 to 89 Days Past Due | Domestic | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0		0
60 to 89 Days Past Due | Domestic | Wealth management loans and mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0		0
60 to 89 Days Past Due | Domestic | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0		0
60 to 89 Days Past Due | Domestic | Other residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	5		3
60 to 89 Days Past Due | Foreign | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	20	[1]	0	[1]
Equal to Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4		6
Equal to Greater than 90 Days Past Due | Domestic
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4		6
Equal to Greater than 90 Days Past Due | Domestic | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0		0
Equal to Greater than 90 Days Past Due | Domestic | Wealth management loans and mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0		1
Equal to Greater than 90 Days Past Due | Domestic | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	0		0
Equal to Greater than 90 Days Past Due | Domestic | Other residential mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	4		5
Equal to Greater than 90 Days Past Due | Foreign | Financial institutions
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total past due	$ 0	[1]	$ 0	[1]

[1] Substantially all of these past due loans have been repaid subsequent to March 31, 2015.