Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 1,134	$ 1,222
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,337	1,409
Allowance for credit losses on TDR Loans		30	48
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[4]	388	482
Unpaid Principal Balance of TDR Loans	[2],[3],[4]	463	548
Allowance for credit losses on TDR Loans		20	37
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	31	32
Unpaid Principal Balance of TDR Loans	[2],[3]	33	33
Allowance for credit losses on TDR Loans		5	0
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	269	300
Unpaid Principal Balance of TDR Loans	[2],[3]	340	363
Allowance for credit losses on TDR Loans		15	37
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	88	150
Unpaid Principal Balance of TDR Loans	[2],[3]	90	152
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	746	740
Unpaid Principal Balance of TDR Loans	[2],[3]	874	861
Allowance for credit losses on TDR Loans		10	11
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	712	708
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	809	797
Allowance for credit losses on TDR Loans		8	9
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	30	27
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	61	59
Allowance for credit losses on TDR Loans		1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	4	5
Unpaid Principal Balance of TDR Loans	[2],[3]	4	5
Allowance for credit losses on TDR Loans		$ 1	$ 1

[1] At March 31, 2017 and December 31, 2016, the carrying value of TDR Loans includes $515 million and $645 million, respectively, of loans which are classified as nonaccrual.

[2] TDR Loans are considered to be impaired loans. For consumer loans, all such loans are considered impaired loans regardless of accrual status. For commercial loans, impaired loans include other loans in addition to TDR Loans which totaled $540 million and $571 million at March 31, 2017 and December 31, 2016, respectively.

[3] The carrying value of TDR Loans includes basis adjustments on the loans, such as unearned income, unamortized deferred fees and costs on originated loans, partial charge-offs and premiums or discounts on purchased loans.

[4] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $232 million and $184 million at March 31, 2017 and December 31, 2016, respectively.

[5] At March 31, 2017 and December 31, 2016, the carrying value of consumer mortgage TDR Loans held for investment includes $681 million and $672 million, respectively, of loans that are recorded at the lower of amortized cost or fair value of the collateral less cost to sell.