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Fair value - Summary of Fair Value Assets Measured on Recurring Basis, Significant Unobservable Input (Details) $ in Thousands	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	$ 5,815,337	$ 5,624,773
U.S. corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	2,900,205	2,825,679
Non-U.S. corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	768,953	772,624
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	111,683	8,364
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	20,702
Fair value, recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	5,815,337	5,624,773
Fair value, recurring | U.S. corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	2,900,205	2,825,679
Fair value, recurring | Non-U.S. corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	768,953	772,624
Fair value, recurring | Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	111,683	8,364
Fair value, recurring | Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	$ 20,702
Level 3  | U.S. corporate | Internal models | Credit spreads | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturity securities, measurement input (in basis points)	0.0014
Level 3  | U.S. corporate | Internal models | Credit spreads | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturity securities, measurement input (in basis points)	0.0174
Level 3  | U.S. corporate | Internal models | Credit spreads | Weighted average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturity securities, measurement input (in basis points)	0.0102
Level 3  | Non-U.S. corporate | Internal models | Credit spreads | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturity securities, measurement input (in basis points)	0.0090
Level 3  | Non-U.S. corporate | Internal models | Credit spreads | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturity securities, measurement input (in basis points)	0.0143
Level 3  | Non-U.S. corporate | Internal models | Credit spreads | Weighted average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturity securities, measurement input (in basis points)	0.0118
Level 3  | Fair value, recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	$ 279,704	281,541
Level 3  | Fair value, recurring | Internal models
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	264,600
Level 3  | Fair value, recurring | U.S. corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	225,970	222,786
Level 3  | Fair value, recurring | U.S. corporate | Internal models
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	225,970
Level 3  | Fair value, recurring | Non-U.S. corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	50,474	56,553
Level 3  | Fair value, recurring | Non-U.S. corporate | Internal models
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	38,603
Level 3  | Fair value, recurring | Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	0	$ 0
Level 3  | Fair value, recurring | Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value	$ 0

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