Contract revenue - Narrative (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Capitalized Contract Cost [Line Items]
Contract with customer, asset	$ 2,500,000,000			$ 2,500,000,000		$ 2,400,000,000
Contract assets representing accrued revenues not yet billed	57,000,000			57,000,000		32,000,000
Contract with customer, liability	187,000,000			187,000,000		168,000,000
Contract with customer, liability, revenue recognized	65,000,000			95,000,000
Capitalized contract cost, impairment				0
Incremental costs for obtaining contracts
Capitalized Contract Cost [Line Items]
Capitalized contract costs	$ 77,000,000			$ 77,000,000		86,000,000
Capitalized contract cost, amortization period	9 years			9 years
Capitalized contract cost, amortization	$ 6,000,000	$ 5,000,000	$ 7,000,000	$ 16,000,000	$ 17,000,000
Costs to fulfill contract
Capitalized Contract Cost [Line Items]
Capitalized contract costs	12,000,000			12,000,000		$ 16,000,000
Capitalized contract cost, amortization	$ 1,000,000	$ 2,000,000	$ 1,000,000	$ 4,000,000	$ 4,000,000
Costs to fulfill contract | Minimum
Capitalized Contract Cost [Line Items]
Capitalized contract cost, amortization period	7 years			7 years
Costs to fulfill contract | Maximum
Capitalized Contract Cost [Line Items]
Capitalized contract cost, amortization period	9 years			9 years