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, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 2,433,785	$ 2,293,296
Financial liabilities	2,220,739	2,099,449
Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	12,388	13,447
Financial liabilities	6,144	5,919
Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	20	25
Asset-backed securities | 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	4,180	3,557
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 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	1,944	2,300
Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	37
Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	184,361	153,691
Trading liabilities | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	184,361	153,691
Trading liabilities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	4,200	3,582	$ 4,285
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	20	25
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 | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	4,180	3,557
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 | 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	94,429	86,832
Financial liabilities designated at fair value | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	94,429	86,832
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	0	37	3
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 | Structured notes | 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 | 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	37
Derivatives | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	216,821	279,819
Derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1,944	2,300	1,210
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 | Structured notes | 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 | 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	1,944	2,300
Derivatives | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0	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	3,508	3,196
Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,577	1,550
Loans held for securitisation | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	24	28
Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3	2
Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	113	175
Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,331	2,577
Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,832	5,919
Available for sale
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	336,157	389,874
Available for sale | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	336,067	389,874
Available for sale | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3,432	3,476	4,727
Available for sale | Private equity including strategic investments | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,012	2,435
Available for sale | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,300	761
Available for sale | 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
Available for sale | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Available for sale | Derivatives with monolines | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Available for sale | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0	0
Available for sale | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	120	280
Trading assets
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	287,995	235,125
Trading assets | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	287,995	235,125
Trading assets | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,052	6,489	6,856
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	38	49
Trading assets | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,986	2,668
Trading assets | Asset-backed securities | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,277	789
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	24	28
Trading assets | Structured notes | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3	2
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	3,710	5,621
Financial assets designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	29,464	24,756
Financial assets designated at fair value | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	29,464	24,756
Financial assets designated at fair value | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,460	730	474
Financial assets 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 assets	1,458	712
Financial assets designated at fair value | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2	20
Financial assets 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 assets	0	0
Financial assets 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 assets	0	0
Financial assets designated at fair value | 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 at fair value | 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 at fair value | 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 at fair value | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2	18
Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	219,818	290,872
Derivatives | Recurring fair value measurement
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	219,818	290,872
Derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,444	2,752	$ 2,262
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	113	175
Derivatives | Other derivatives | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,331	2,577
Derivatives | Other portfolios | Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 0	$ 0