Fair value measurements - Schedule of Investments in Company-Sponsored Funds (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 5,619,000	$ 5,373
Unfunded Commitments	790	1,314
Hedge Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	141,000	283
Unfunded Commitments	$ 0	$ 0
Hedge Funds | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period	30 days	30 days
Hedge Funds | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Redemption Notice Period	120 days	120 days
Private Equity Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 5,478,000	$ 5,090
Unfunded Commitments	$ 790	$ 1,314