Loans - Trouble Debt Restructuring Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 1,243	$ 1,204	$ 1,229	$ 1,250
Interest income recognized on TDR loans	11	11	22	26
Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	218	248	215	295
Interest income recognized on TDR loans	1	2	3	8
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	149	220	161	244
Interest income recognized on TDR loans	1	2	2	8
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	56	23	45	22
Interest income recognized on TDR loans	0	0	1	0
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	13	0	9	17
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	0	5	0	12
Consumer Loans [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	1,025	956	1,014	955
Interest income recognized on TDR loans	10	9	19	18
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	999	929	988	928
Interest income recognized on TDR loans	9	8	18	17
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	20	19	20	19
Interest income recognized on TDR loans	1	1	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Modifications [Line Items]
Average balance of TDR loans	$ 6	$ 8	$ 6	$ 8