Fair value measurements - Schedule of Realized and Unrealized Gains From Investments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	$ (1,546)	$ (296)
Unrealized gain/(loss) on investments	(12,036)	1,332
Asset Management
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	367	60
Unrealized gain/(loss) on investments	(723)	781
Net gains (losses) from investment activities	(351)	841
Asset Management | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized gain/(loss) on investments	0	968
Net gains (losses) from investment activities	0	968
Asset Management | Debt obligation | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized gain/(loss) on investments	0	968
Net gains (losses) from investment activities	0	968
Asset Management | Corporate loans
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	0	0
Unrealized gain/(loss) on investments	0	0
Net gains (losses) from investment activities	5	0
Insurance Solutions | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	(9)	(194)
Unrealized gain/(loss) on investments	(4,992)	1,756
Net gains (losses) from investment activities	(5,001)	1,562
Insurance Solutions | Variable Interest Entity, Primary Beneficiary
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	(706)	60
Unrealized gain/(loss) on investments	(2,971)	(326)
Net gains (losses) from investment activities	(3,677)	(266)
Insurance Solutions | Variable Interest Entity, Primary Beneficiary | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	(706)	60
Unrealized gain/(loss) on investments	(2,971)	(326)
Net gains (losses) from investment activities	(3,677)	(266)
Insurance Solutions | Consolidated Balance Including Variable Interest Entity
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	(1,913)	(356)
Unrealized gain/(loss) on investments	(11,313)	1,520
Net gains (losses) from investment activities	(8,691)	1,206
Insurance Solutions | Consolidated Balance Including Variable Interest Entity | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	(715)	(134)
Unrealized gain/(loss) on investments	(7,963)	1,430
Net gains (losses) from investment activities	(8,678)	1,296
Insurance Solutions | U.S. state, territories and municipalities | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized gain/(loss) on investments	(15)	57
Net gains (losses) from investment activities	(15)	57
Insurance Solutions | Other government and agency
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	0	0
Unrealized gain/(loss) on investments	(28)	(1)
Net gains (losses) from investment activities	(28)	(1)
Insurance Solutions | Other government and agency | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized gain/(loss) on investments	(28)	(1)
Net gains (losses) from investment activities	(28)	(1)
Insurance Solutions | Corporate
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	32	(10)
Unrealized gain/(loss) on investments	(2,404)	1,186
Net gains (losses) from investment activities	(2,372)	1,176
Insurance Solutions | Corporate | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments		(50)
Unrealized gain/(loss) on investments	(2,404)	1,186
Net gains (losses) from investment activities	(2,404)	1,136
Insurance Solutions | Asset and mortgage-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	(210)	(158)
Unrealized gain/(loss) on investments	(2,179)	115
Net gains (losses) from investment activities	(2,389)	(43)
Insurance Solutions | Asset and mortgage-backed securities | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	(9)	(146)
Unrealized gain/(loss) on investments	(2,179)	115
Net gains (losses) from investment activities	(2,188)	(31)
Insurance Solutions | Corporate loans
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments	0	2
Unrealized gain/(loss) on investments	(400)	789
Net gains (losses) from investment activities	(400)	791
Insurance Solutions | Corporate loans | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments		2
Unrealized gain/(loss) on investments	(400)	789
Net gains (losses) from investment activities	(400)	791
Insurance Solutions | Other invested assets | Fair value option
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized gain/(loss) on investments		0
Unrealized gain/(loss) on investments	34	(390)
Net gains (losses) from investment activities	$ 34	$ (390)