Fair values of financial instruments carried at fair value - Financial instruments measured at fair value using a valuation technique with significant unobservable inputs - Level 3 (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 2,471,203		$ 2,433,785
Financial liabilities	2,260,288		2,220,739
Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	16,674		12,388
Financial liabilities	7,142		6,144
Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	12		20
Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,374		4,180
Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1,755		1,944
Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1		0
Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	84,431		184,361
Trading liabilities | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	84,431		184,361
Trading liabilities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	58	$ 93	4,200	$ 3,582
Trading liabilities | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	12		20
Trading liabilities | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Trading liabilities | Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Trading liabilities | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	46		4,180
Trading liabilities | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Trading liabilities | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Trading liabilities | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Financial liabilities designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	148,505		94,429
Financial liabilities designated at fair value | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	148,505		94,429
Financial liabilities designated at fair value | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,328	4,107	0	37
Financial liabilities designated at fair value | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Financial liabilities designated at fair value | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Financial liabilities designated at fair value | Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Financial liabilities designated at fair value | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	5,328		0
Financial liabilities designated at fair value | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Financial liabilities designated at fair value | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Financial liabilities designated at fair value | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Derivatives | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	205,835		216,821
Derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1,756	1,949	1,944	2,300
Derivatives | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Derivatives | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Derivatives | Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Derivatives | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Derivatives | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Derivatives | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1,755		1,944
Derivatives | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1		0
Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,553		3,508
Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,202		2,577
Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	49		24
Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3		3
Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	65		113
Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,358		2,331
Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	6,444		3,832
Financial investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets			336,157
Financial investments | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	344,767		336,067
Financial investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,000	1,767	3,432	3,476
Financial investments | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	427		2,012
Financial investments | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,030		1,300
Financial investments | Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Financial investments | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Financial investments | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Financial investments | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Financial investments | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	543		120
Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	238,130		287,995
Trading assets | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	238,130		287,995
Trading assets | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	6,759	5,080	5,052	6,489
Trading assets | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	20		38
Trading assets | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,351		2,986
Trading assets | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,140		1,277
Trading assets | Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		24
Trading assets | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3		3
Trading assets | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Trading assets | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Trading assets | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,596		3,710
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,349		29,464
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	41,111		29,464
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,492	3,958	1,460	730
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,106		1,458
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		2
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	32		0
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	49		0
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Financial assets designated and otherwise mandatorily measured at fair value through profit or loss | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	305		2
Derivatives | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	207,825		219,818
Derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,423	$ 2,444	2,444	$ 2,752
Derivatives | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Derivatives | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Derivatives | Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Derivatives | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Derivatives | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	65		113
Derivatives | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,358		2,331
Derivatives | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 0		$ 0