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Fair value - Summary of Significant Inputs used for Fair Value Measurement of Fixed Maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 5,815,337	$ 5,624,773
State and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	476,141	467,476
Non-U.S. government
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	113,153	83,802
U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	2,900,205	2,825,679
Non-U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	768,953	772,624
Residential mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	111,683	8,364
Commercial mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	20,702
Other asset-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	1,135,492	1,189,465
Fair value, recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	5,815,337	5,624,773
Fair value, recurring | U.S. government, agencies and GSEs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	289,008	277,363
Fair value, recurring | State and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	476,141	467,476
Fair value, recurring | Non-U.S. government
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	113,153	83,802
Fair value, recurring | U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	2,900,205	2,825,679
Fair value, recurring | Non-U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	768,953	772,624
Fair value, recurring | Residential mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	111,683	8,364
Fair value, recurring | Commercial mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	20,702
Fair value, recurring | Other asset-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	1,135,492	1,189,465
Level 2 | Fair value, recurring
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	5,535,633	5,343,232
Level 2 | Fair value, recurring | U.S. government, agencies and GSEs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	289,008	277,363
Level 2 | Fair value, recurring | State and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	476,141	467,476
Level 2 | Fair value, recurring | Non-U.S. government
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	113,153	83,802
Level 2 | Fair value, recurring | U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	2,674,235	2,602,893
Level 2 | Fair value, recurring | Non-U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	718,479	716,071
Level 2 | Fair value, recurring | Residential mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	111,683	8,364
Level 2 | Fair value, recurring | Commercial mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	20,702
Level 2 | Fair value, recurring | Other asset-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	1,132,232	$ 1,187,263
Pricing Services | Level 2 | Fair value, recurring | U.S. government, agencies and GSEs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	289,008
Pricing Services | Level 2 | Fair value, recurring | State and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	476,141
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. government
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	113,153
Pricing Services | Level 2 | Fair value, recurring | U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	2,483,692
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. corporate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	644,100
Pricing Services | Level 2 | Fair value, recurring | Residential mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	111,683
Pricing Services | Level 2 | Fair value, recurring | Commercial mortgage-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	20,702
Pricing Services | Level 2 | Fair value, recurring | Other asset-backed
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 1,132,232

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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