Stockholders' equity (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Class of Stock [Line Items]
Stock options outstanding (shares)	3,431,000	3,963,000
Common stock available for future grants (shares)	48,184,000
Stockholders' Equity Note, Outstanding Shares Less than 10% of Aggregate Shares Outstanding, Conversion Ratio	1
Stockholders' Equity Attributable to Parent | $	$ 216,018	$ 233,529	$ 212,112	$ 298,705
Treasury Stock [Member]
Class of Stock [Line Items]
Stockholders' Equity Attributable to Parent | $	$ (113,613)	$ (113,613)	$ (113,613)	$ (113,613)
Common Class A [Member]
Class of Stock [Line Items]
Common stock authorized (shares)	500,000,000	500,000,000
Common stock outstanding (shares)	122,233,000	117,922,000
Common Stock, Voting Rights, Number	1
Common Stock, Conversion Ratio	1
Common Stock, Shares, Issued	122,233,000	117,922,000
Common Stock, Voting Rights	one
Common Stock, Voting Rights	one
Common Class B [Member]
Class of Stock [Line Items]
Common stock authorized (shares)	150,000,000	150,000,000
Common stock outstanding (shares)	28,885,000	28,897,000
Common Stock, Voting Rights, Number	10
Common Stock, Shares, Issued	28,885,000	28,897,000
Common Stock, Voting Rights	ten
Common Stock, Voting Rights	ten
Restricted Stock Units (RSUs) [Member]
Class of Stock [Line Items]
Restricted stock units outstanding (shares)	10,639,000	8,225,000
Performance Shares [Member]
Class of Stock [Line Items]
Restricted stock units outstanding (shares)	1,319,000	788,000
Common Stock
Class of Stock [Line Items]
Common stock available for future grants (shares)	32,795,000