v3.25.2
Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value $ 3,918,078 $ 3,537,563
U.S. Treasury securities and obligations of U.S. government agencies    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 7,610 15,048
Obligations of states, municipalities and political subdivisions [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 137,580 146,304
Corporate and other securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 2,100,034 1,989,490
Asset-backed securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 783,907 732,742
Residential mortgage-backed securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 563,791 448,874
Commercial mortgage-backed securities    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 325,156 205,105
Recurring    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 4,466,270 3,939,636
Recurring | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 528,874 390,688
Recurring | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 3,937,396 3,548,948
Recurring | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Fixed maturities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 3,918,078 3,537,563
Recurring | Fixed maturities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 7,610 15,048
Recurring | Fixed maturities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 3,910,468 3,522,515
Recurring | Fixed maturities [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | U.S. Treasury securities and obligations of U.S. government agencies    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 7,610 15,048
Recurring | U.S. Treasury securities and obligations of U.S. government agencies | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 7,610 15,048
Recurring | U.S. Treasury securities and obligations of U.S. government agencies | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | U.S. Treasury securities and obligations of U.S. government agencies | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Obligations of states, municipalities and political subdivisions [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 137,580 146,304
Recurring | Obligations of states, municipalities and political subdivisions [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Obligations of states, municipalities and political subdivisions [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 137,580 146,304
Recurring | Obligations of states, municipalities and political subdivisions [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Corporate and other securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 2,100,034 1,989,490
Recurring | Corporate and other securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Corporate and other securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 2,100,034 1,989,490
Recurring | Corporate and other securities [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Asset-backed securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 783,907 732,742
Recurring | Asset-backed securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Asset-backed securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 783,907 732,742
Recurring | Asset-backed securities [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Residential mortgage-backed securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 563,791 448,874
Recurring | Residential mortgage-backed securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Residential mortgage-backed securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 563,791 448,874
Recurring | Residential mortgage-backed securities [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Commercial mortgage-backed securities    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 325,156 205,105
Recurring | Commercial mortgage-backed securities | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Commercial mortgage-backed securities | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 325,156 205,105
Recurring | Commercial mortgage-backed securities | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 0 0
Recurring | Equity securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 513,882 398,359
Recurring | Equity securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 486,954 371,926
Recurring | Equity securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 26,928 26,433
Recurring | Equity securities [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Exchange traded funds [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 149,441 129,731
Recurring | Exchange traded funds [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 149,441 129,731
Recurring | Exchange traded funds [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Exchange traded funds [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Nonredeemable preferred stock [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 26,928 26,433
Recurring | Nonredeemable preferred stock [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Nonredeemable preferred stock [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 26,928 26,433
Recurring | Nonredeemable preferred stock [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Common Stock [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 337,513 242,195
Recurring | Common Stock [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 337,513 242,195
Recurring | Common Stock [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Common Stock [Member] | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Short-term Investments    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 34,310 3,714
Recurring | Short-term Investments | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 34,310 3,714
Recurring | Short-term Investments | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 0 0
Recurring | Short-term Investments | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value $ 0 $ 0