Summary of significant accounting policies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($) segment	Dec. 31, 2022 USD ($) segment
Concentration Risk [Line Items]
Number of operating segments | segment	2	2	2
Number of reportable segments | segment	2	2	2
Consumer subscription trail period	7 days
Refund term	30 days
Advertising cost | $	$ 94.9	$ 82.5	$ 97.0
Amortization period	4 years
Self insurance | $	$ 1.1	$ 1.3
Minimum
Concentration Risk [Line Items]
Consumer subscription term	1 month
Maximum
Concentration Risk [Line Items]
Consumer subscription term	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Concentration Risk [Line Items]
Revenue, remaining performance obligation, period	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Concentration Risk [Line Items]
Revenue, remaining performance obligation, period
Consumer Single Course Purchases | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Concentration Risk [Line Items]
Revenue, remaining performance obligation, period	4 months
Computers and equipment
Concentration Risk [Line Items]
Estimated useful lives	3 years
Purchased software
Concentration Risk [Line Items]
Estimated useful lives	3 years
Equipment
Concentration Risk [Line Items]
Estimated useful lives	3 years
Furniture and fixtures
Concentration Risk [Line Items]
Estimated useful lives	5 years
Software Development
Concentration Risk [Line Items]
Estimated useful lives	3 years