Fair value measurements - Assets and Liabilities on Recurring Basis (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Transfers In Fair Value Hierarchy	0
Investments	$ 1,876,564	$ 1,611,114
Investments in publicly traded companies
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in publicly traded companies	483,817	559,830
Investments in publicly traded companies | Fair value measured on recurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in publicly traded companies	483,817	559,830
Investments in publicly traded companies | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measured on recurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in publicly traded companies	483,817	559,830
Investments in publicly traded companies | Significant Other Observable Inputs (Level 2) | Fair value measured on recurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in publicly traded companies	0	0
Investments in publicly traded companies | Significant Unobservable Inputs (Level 3) | Fair value measured on recurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in publicly traded companies	0	0
Investments in privately held entities that report NAV
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in publicly traded companies	829,864	662,082
Investments in privately held entities that do not report NAV | Entities without observable price changes
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in privately held entities that do not report fair value, book value	362,064	244,798
Investments in privately held entities that do not report NAV | Entities without observable price changes | Fair Value | Fair value measured on nonrecurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in privately held entities that do not report fair value, book value	138,011	157,871
Investments in privately held entities that do not report NAV | Quoted Prices in Active Markets for Identical Assets (Level 1) | Entities without observable price changes | Fair Value | Fair value measured on nonrecurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in privately held entities that do not report fair value, book value	0	0
Investments in privately held entities that do not report NAV | Significant Other Observable Inputs (Level 2) | Entities without observable price changes | Fair Value | Fair value measured on nonrecurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in privately held entities that do not report fair value, book value	129,231	144,404
Investments in privately held entities that do not report NAV | Significant Unobservable Inputs (Level 3) | Entities without observable price changes | Fair Value | Fair value measured on nonrecurring basis
Assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy
Investment in privately held entities that do not report fair value, book value	$ 8,780	$ 13,467