Contract revenue - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Capitalized Contract Cost [Line Items]
Contract with customer, asset	$ 2,600,000,000	$ 2,400,000,000
Contract assets representing accrued revenues not yet billed	48,000,000	32,000,000
Contract with customer, liability	194,000,000	168,000,000
Contract with customer, liability, revenue recognized	50,000,000
Capitalized contract cost, impairment	0
Incremental costs for obtaining contracts
Capitalized Contract Cost [Line Items]
Capitalized contract costs	$ 89,000,000	86,000,000
Capitalized contract cost, amortization period	9 years
Capitalized contract cost, amortization	$ 5,000,000	5,000,000	$ 5,000,000
Costs to fulfill contract
Capitalized Contract Cost [Line Items]
Capitalized contract costs	14,000,000	16,000,000
Capitalized contract cost, amortization	$ 1,000,000	$ 1,000,000	$ 1,000,000
Costs to fulfill contract | Minimum
Capitalized Contract Cost [Line Items]
Capitalized contract cost, amortization period	7 years
Costs to fulfill contract | Maximum
Capitalized Contract Cost [Line Items]
Capitalized contract cost, amortization period	9 years