Summary of significant accounting policies - Narrative (Details) $ / shares in Units, $ in Thousands		12 Months Ended			108 Months Ended
	Jan. 20, 2021 USD ($) shares	Dec. 31, 2021 USD ($) segment shares	Dec. 31, 2020 USD ($) segment shares	Dec. 31, 2019 USD ($) segment shares	Dec. 31, 2018 USD ($) shares	Sep. 30, 2011	Jan. 31, 2010 $ / shares shares
Disaggregation of Revenue [Line Items]
Number of operating segments | segment		2	2	2
Number of reportable segments | segment		2	2	2
Consumer subscription duration		1 month
Refund term		30 days
Assets		$ 739,851	$ 282,096
Adjustment in connection with accounting standard adoption		389,700	(260,685)	$ (225,590)	$ (183,568)
Advertising costs		$ 105,200	$ 110,500	$ 72,600
Expected dividend yield		0.00%
Number of shares called by warrants (in shares) | shares							1,562,689
Exercise price of warrants (in dollars per share) | $ / shares							$ 0.196
Warrant term (in years)						5 years
Warrant extension term (in years)						5 years
Warrants exercised (in shares) | shares	12,595				1,550,094
Warrants outstanding (in shares) | shares		0	12,595	12,595
Fair value amount of warrants reclassified	$ 200
Accumulated Deficit
Disaggregation of Revenue [Line Items]
Adjustment in connection with accounting standard adoption		$ (458,529)	$ (378,503)	$ (300,883)	$ (237,913)
Cumulative Effect, Period of Adoption, Adjustment
Disaggregation of Revenue [Line Items]
Adjustment in connection with accounting standard adoption					6,733
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Assets					6,700
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-02 | Minimum | Scenario, Plan
Disaggregation of Revenue [Line Items]
Operating lease, ROU assets		16,500
Operating lease liabilities		19,000
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-02 | Maximum | Scenario, Plan
Disaggregation of Revenue [Line Items]
Operating lease, ROU assets		19,000
Operating lease liabilities		$ 21,000
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Deficit
Disaggregation of Revenue [Line Items]
Adjustment in connection with accounting standard adoption					6,733
Cumulative Effect, Period of Adoption, Adjustment | Accumulated Deficit | Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Adjustment in connection with accounting standard adoption					$ 6,700