Net loss per share Additional Information (Details) - USD ($) $ in Millions		3 Months Ended
	Apr. 12, 2017	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Document Period End Date		Mar. 31, 2020
Treasury Shares Acquired, Estimated, Prepaid Forward	9,200,000	9,200,000	9,200,000
Common Class A [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common Stock, Voting Rights, Number		1
Conversion of Stock, Shares Issued		1
Common Class B [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common Stock, Voting Rights, Number		10
Convertible Debt [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt Instrument	$ 175.0
Interest rate	3.50%
Maximum number of shares issuable upon conversion of the notes	20,600,000