Fair value measurement (Details) - USD ($)	3 Months Ended	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on investments	$ 1,600,000	$ 0	$ 0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of total long-term debt		$ 3,990,000,000		$ 4,320,000,000