Loans - Summary of Criticized Assets for Commercial Loans (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 83,617	$ 77,741	$ 72,198
Commercial Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	63,596	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	11,520	10,300	9,682
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	19,712	17,819	15,956
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	28,942	26,387	24,268
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,422	3,581	$ 3,127
Commercial Loans [Member] | Criticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,200	3,787
Commercial Loans [Member] | Criticized [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	373	547
Commercial Loans [Member] | Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,511	1,261
Commercial Loans [Member] | Criticized [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,306	1,972
Commercial Loans [Member] | Criticized [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	10	7
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,308	3,082
Commercial Loans [Member] | Special Mention [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	158	310
Commercial Loans [Member] | Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,060	1,001
Commercial Loans [Member] | Special Mention [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,088	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	885	676
Commercial Loans [Member] | Substandard [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	213	230
Commercial Loans [Member] | Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	447	238
Commercial Loans [Member] | Substandard [Member] | Global Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	218	202
Commercial Loans [Member] | Substandard [Member] | Other Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7	6
Commercial Loans [Member] | Doubtful [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	7	29
Commercial Loans [Member] | Doubtful [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	2	7
Commercial Loans [Member] | Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Recorded Investment [Line Items]
Total loans	4	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	$ 1	$ 0