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Employee benefit plans - Pension Plan Assets Valued Using Net Asset Value (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Minimum | Funds of funds    
Defined Benefit Plan Disclosure [Line Items]    
Redemption notice period 30 days 30 days
Minimum | Property funds    
Defined Benefit Plan Disclosure [Line Items]    
Redemption notice period 0 days 0 days
Maximum | Funds of funds    
Defined Benefit Plan Disclosure [Line Items]    
Redemption notice period 45 days 45 days
Maximum | Property funds    
Defined Benefit Plan Disclosure [Line Items]    
Redemption notice period 90 days 90 days
NAV    
Defined Benefit Plan Disclosure [Line Items]    
Fair value $ 320 $ 382
Unfunded commitments 0 0
NAV | Funds of funds    
Defined Benefit Plan Disclosure [Line Items]    
Fair value 136 147
Unfunded commitments $ 0 $ 0
Redemption frequency Monthly Monthly
NAV | Venture capital and partnership interests    
Defined Benefit Plan Disclosure [Line Items]    
Fair value $ 158 $ 148
Unfunded commitments 0 0
NAV | Property funds    
Defined Benefit Plan Disclosure [Line Items]    
Fair value 4 52
Unfunded commitments $ 0 $ 0
Redemption frequency Monthly Monthly
NAV | Corporate debt    
Defined Benefit Plan Disclosure [Line Items]    
Fair value   $ 19
Unfunded commitments   0
NAV | Other contracts    
Defined Benefit Plan Disclosure [Line Items]    
Fair value $ 22 16
Unfunded commitments $ 0 $ 0