FVM - Level 3 instruments: Valuation techniques and inputs (Detail) $ in Millions		3 Months Ended
		Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2021 USD ($)
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		$ 345,820	$ 404,805	$ 376,304
Financial liabilities measured at fair value		299,869	325,069	324,088
Level 3
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		8,129	8,278	8,266
Financial liabilities measured at fair value		18,398	15,212	$ 16,355
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Corporate and municipal bonds
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		1,200	1,200
Financial liabilities measured at fair value		$ 0	$ 0
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Corporate and municipal bonds | Relative value to market comparable | Bond price equivalent (% of par) | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0015	0.0001
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Corporate and municipal bonds | Relative value to market comparable | Bond price equivalent (% of par) | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0143	0.0143
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Corporate and municipal bonds | Relative value to market comparable | Bond price equivalent (% of par) | weighted average
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1],[2]	0.0100	0.0100
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Corporate and municipal bonds | Discounted expected cash flows | Discount margin | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0358	0.0268
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Corporate and municipal bonds | Discounted expected cash flows | Discount margin | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0358	0.0268
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		$ 2,600	$ 2,400
Financial liabilities measured at fair value		$ 0	$ 0
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Relative value to market comparable | Loan price equivalent (% of par) | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0001	0
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Relative value to market comparable | Loan price equivalent (% of par) | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0101	0.0101
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Relative value to market comparable | Loan price equivalent (% of par) | weighted average
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1],[2]	0.0099	0.0099
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Discounted expected cash flows | Credit spread | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0180	0.0190
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Discounted expected cash flows | Credit spread | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0800	0.0800
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Market comparable and securitization model | Credit spread | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0028	0.0040
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Market comparable and securitization model | Credit spread | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.1558	0.1858
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Market comparable and securitization model | Credit spread | weighted average
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1],[2]	0.0228	0.0333
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Market comparable and securitization model | Discount margin | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0115	0.0100
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Market comparable and securitization model | Discount margin | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0222	0.0188
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Traded loans, loans mandatorily at fair value, loan commitments and guarantees | Market comparable and securitization model | Discount margin | weighted average
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1],[2]	0.0162	0.0140
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Auction rate securities
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		$ 1,600	$ 1,500
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Auction rate securities | Discounted expected cash flows | Credit spread | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0115	0.0100
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Auction rate securities | Discounted expected cash flows | Credit spread | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0222	0.0188
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Auction rate securities | Discounted expected cash flows | Credit spread | weighted average
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1],[2]	0.0162	0.0140
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Investment fund units
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value	[3]	$ 100	$ 100
Financial liabilities measured at fair value	[3]	0	0
Level 3 | Financial assets and liabilities at fair value held for trading, Financial assets at fair value not held for trading: Equity instruments
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value	[3]	700	700
Financial liabilities measured at fair value	[3]	100	0
Level 3 | Debt issued designated at fair value
Disclosure Of Fair Value Measurement [Line Items]
Financial liabilities measured at fair value	[4]	14,700	11,000
Level 3 | Other financial liabilities designated at fair value
Disclosure Of Fair Value Measurement [Line Items]
Financial liabilities measured at fair value		$ 600	$ 700
Level 3 | Other financial liabilities designated at fair value | Discounted expected cash flows | Funding spread | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0035	0.0042
Level 3 | Other financial liabilities designated at fair value | Discounted expected cash flows | Funding spread | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0175	0.0175
Level 3 | Derivative financial instruments: Interest rate contracts
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		$ 300	$ 500
Financial liabilities measured at fair value		$ 500	$ 500
Level 3 | Derivative financial instruments: Interest rate contracts | Option model | Volatility of interest rates | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0049	0.0029
Level 3 | Derivative financial instruments: Interest rate contracts | Option model | Volatility of interest rates | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0073	0.0069
Level 3 | Derivative financial instruments: Credit derivative contracts
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		$ 300	$ 300
Financial liabilities measured at fair value		$ 400	$ 500
Level 3 | Derivative financial instruments: Credit derivative contracts | Discounted expected cash flows | Bond price equivalent (% of par) | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.03	0
Level 3 | Derivative financial instruments: Credit derivative contracts | Discounted expected cash flows | Bond price equivalent (% of par) | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	1.02	1
Level 3 | Derivative financial instruments: Credit derivative contracts | Discounted expected cash flows | Credit spread | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0002	0.0001
Level 3 | Derivative financial instruments: Credit derivative contracts | Discounted expected cash flows | Credit spread | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.0496	0.0489
Level 3 | Derivative financial instruments: Equity / index contracts
Disclosure Of Fair Value Measurement [Line Items]
Financial assets measured at fair value		$ 800	$ 900
Financial liabilities measured at fair value		$ 1,900	$ 2,300
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Equity dividend yields | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0	0
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Equity dividend yields | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)	[1]	0.11	0.13
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Volatility of equity stocks, equity and other indices | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)		0.04	0.04
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Volatility of equity stocks, equity and other indices | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)		0.99	1
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Equity-to-FX correlation | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)		(0.30)	(0.34)
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Equity-to-FX correlation | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)		0.70	0.65
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Equity-to-equity correlation | low
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)		(0.25)	(0.16)
Level 3 | Derivative financial instruments: Equity / index contracts | Option model | Equity-to-equity correlation | high
Disclosure Of Fair Value Measurement [Line Items]
Significant unobservable input(s)		0.99	1

[1] <div>The ranges of significant unobservable inputs are represented in points, percentages and basis points. Points are a percentage of par (e.g., 100 points would be 100% of par)</div>

[2] <div>Weighted averages are provided for non-derivative financial instruments and were calculated by weighting inputs based on the fair values of the respective instruments. Weighted averages are not provided for inputs related to derivative contracts, as this would not be meaningful.</div>

[3] <div>The range of inputs is not disclosed, as there is a dispersion of values given the diverse nature of the investments.</div>

[4] <div>Debt issued designated at fair value is composed primarily of UBS structured notes, which include variable maturity notes with various equity and foreign exchange underlying risks, rates-linked and credit-linked notes, all of which have embedded derivative parameters that are considered to be unobservable. The equivalent derivative instrument parameters are presented in the respective derivative financial instruments lines in this table.</div>