v3.26.3
Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value $ 4,497,325 $ 4,341,450
U.S. Treasury securities and obligations of U.S. government agencies    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 838 874
Obligations of states, municipalities and political subdivisions [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 78,752 127,860
Corporate and other securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 1,732,012 1,713,888
Asset-backed securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 531,269 588,519
Residential mortgage-backed securities [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 1,655,788 1,459,270
Commercial mortgage-backed securities    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 498,666 451,039
Recurring    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 5,276,558 4,971,713
Recurring | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 767,283 607,037
Recurring | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 4,509,275 4,364,676
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 4,497,325 4,341,450
Recurring | Fixed maturities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 838 874
Recurring | Fixed maturities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Estimated Fair Value 4,496,487 4,340,576
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 838 874
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 838 874
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 78,752 127,860
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 78,752 127,860
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 1,732,012 1,713,888
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 1,732,012 1,713,888
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 531,269 588,519
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 531,269 588,519
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 1,655,788 1,459,270
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 1,655,788 1,459,270
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 498,666 451,039
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 498,666 451,039
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 779,233 626,399
Recurring | Equity securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 766,445 602,299
Recurring | Equity securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 12,788 24,100
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 240,265 185,523
Recurring | Exchange traded funds [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 240,265 185,523
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 12,788 24,100
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 12,788 24,100
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 526,180 416,776
Recurring | Common Stock [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value 526,180 416,776
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   3,864
Recurring | Short-term Investments | Fair Value, Inputs, Level 1 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value   3,864
Recurring | Short-term Investments | Fair Value, Inputs, Level 2 [Member]    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value   0
Recurring | Short-term Investments | Level 3    
Fair Value of Investments Measured on Recurring Basis [Line Items]    
Fair value   $ 0