Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 84,942	$ 77,741	$ 73,439
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	64,752	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	12,096	10,300	9,661
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	19,514	17,819	17,355
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	29,808	26,387	24,001
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,334	3,581	$ 3,045
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,003	3,787
Commercial Loans [Member] | Criticized [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	312	547
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,625	1,261
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,058	1,972
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	8	7
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,576	3,082
Commercial Loans [Member] | Special Mention [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	119	310
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,031	1,001
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,424	1,770
Commercial Loans [Member] | Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2	1
Commercial Loans [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,411	676
Commercial Loans [Member] | Substandard [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	193	230
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	578	238
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	634	202
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	6	6
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	16	29
Commercial Loans [Member] | Doubtful [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	7
Commercial Loans [Member] | Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	16	22
Commercial Loans [Member] | Doubtful [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial Loans [Member] | Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0