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Risk management and financial instruments - Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Assets    
Restricted cash $ 216 $ 197
Fair value, recurring basis | Level 1 | Carrying value    
Assets    
Cash and cash equivalents 428 526
Restricted cash 216 197
Marketable securities 13 8
Fair value, recurring basis | Level 1 | Fair value    
Assets    
Cash and cash equivalents 428 526
Restricted cash 216 197
Marketable securities 13 8
Fair value, recurring basis | Level 3    
Assets    
Related party loans receivable   85
Fair value, recurring basis | Level 3 | Carrying value    
Assets    
Related party loans receivable 19 13
Fair value, recurring basis | Level 3 | Fair value    
Assets    
Related party loans receivable $ 19 $ 13