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	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 323,199
Available for sale
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets			$ 336,157
Recurring fair value measurement | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	83,845		184,361
Recurring fair value measurement | Financial liabilities designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	151,985		94,429
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	222,961		216,821
Recurring fair value measurement | Available for sale
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	323,681		336,067
Recurring fair value measurement | Trading
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	247,892		287,995
Recurring fair value measurement | Financial assets designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	40,678		29,464
Recurring fair value measurement | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	227,972		219,818
Recurring fair value measurement | Level 3
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	16,251		12,388
Financial liabilities	6,300		6,144
Recurring fair value measurement | Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	17		20
Recurring fair value measurement | Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	4,413		4,180
Recurring fair value measurement | Level 3 | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1,869		1,944
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1		0
Recurring fair value measurement | Level 3 | Trading liabilities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	75	$ 93	4,200	$ 4,050	$ 3,582
Recurring fair value measurement | Level 3 | Trading liabilities | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	17		20
Recurring fair value measurement | Level 3 | Trading liabilities | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Trading liabilities | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Trading liabilities | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	58		4,180
Recurring fair value measurement | Level 3 | Trading liabilities | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Trading liabilities | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Trading liabilities | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	4,355	4,107	0	1	37
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	4,355		0
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Financial liabilities designated at fair value | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1,870	1,949	1,944	2,063	2,300
Recurring fair value measurement | Level 3 | Derivatives | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Derivatives | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Derivatives | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Derivatives | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Derivatives | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	0		0
Recurring fair value measurement | Level 3 | Derivatives | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1,869		1,944
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial liabilities	1		0
Recurring fair value measurement | Level 3 | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	4,872		3,508
Recurring fair value measurement | Level 3 | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,249		2,577
Recurring fair value measurement | Level 3 | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	50		24
Recurring fair value measurement | Level 3 | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3		3
Recurring fair value measurement | Level 3 | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	75		113
Recurring fair value measurement | Level 3 | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,395		2,331
Recurring fair value measurement | Level 3 | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	6,607		3,832
Recurring fair value measurement | Level 3 | Available for sale
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,228	1,767	3,432	3,449	3,476
Recurring fair value measurement | Level 3 | Available for sale | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	457		2,012
Recurring fair value measurement | Level 3 | Available for sale | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,022		1,300
Recurring fair value measurement | Level 3 | Available for sale | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Available for sale | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Available for sale | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Available for sale | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Available for sale | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	749		120
Recurring fair value measurement | Level 3 | Trading
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	6,547	5,080	5,052	4,804	6,489
Recurring fair value measurement | Level 3 | Trading | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	27		38
Recurring fair value measurement | Level 3 | Trading | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	1,223		1,277
Recurring fair value measurement | Level 3 | Trading | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		24
Recurring fair value measurement | Level 3 | Trading | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	3		3
Recurring fair value measurement | Level 3 | Trading | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Trading | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Trading | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,294		3,710
Recurring fair value measurement | Level 3 | Financial assets designated at fair value
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	5,006	3,957	1,460	1,067	730
Recurring fair value measurement | Level 3 | Financial assets designated at fair value | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	4,388		1,458
Recurring fair value measurement | Level 3 | Financial assets designated at fair value | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	4		0
Recurring fair value measurement | Level 3 | Financial assets designated at fair value | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	50		0
Recurring fair value measurement | Level 3 | Financial assets designated at fair value | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Financial assets designated at fair value | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Financial assets designated at fair value | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Financial assets designated at fair value | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	564		2
Recurring fair value measurement | Level 3 | Derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,470	$ 2,444	2,444	$ 2,669	$ 2,752
Recurring fair value measurement | Level 3 | Derivatives | Private equity including strategic investments
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Derivatives | Asset-backed securities
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Derivatives | Loans held for securitisation
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Derivatives | Structured notes
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	0		0
Recurring fair value measurement | Level 3 | Derivatives | Derivatives with monolines
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	75		113
Recurring fair value measurement | Level 3 | Derivatives | Other derivatives
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	2,395		2,331
Recurring fair value measurement | Level 3 | Derivatives | Other portfolios
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Line Items]
Financial assets	$ 0		$ 0