Fair values of pension plan assets - Level 3 rolforward (Details) - Guaranteed Deposit Account [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan investment sales	$ 11	$ 12
Defined Benefit Plan, Plan Assets, Amount	5	12
Defined benefit plan investment purchases	4	15
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Amount	$ 5	$ 12	$ 9