Investments - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 17,504,967	$ 17,155,012
Percentage of total	100.00%	100.00%
U.S. Government direct, guaranteed, and government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 375,979	$ 358,960
States, municipalities, and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	2,760,981	2,786,804
Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	28,381	28,031
Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	14,207,780	13,861,261
Corporates | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	7,385,250	7,182,256
Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	4,762,202	4,675,930
Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	2,060,328	2,003,075
Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	41,655	42,866
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	90,191	77,090
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 0	$ 0
Percentage of total	0.00%	0.00%
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government direct, guaranteed, and government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 0	$ 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | States, municipalities, and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 17,013,533	$ 16,680,898
Percentage of total	97.00%	97.00%
Significant Other Observable Inputs (Level 2) | U.S. Government direct, guaranteed, and government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 375,979	$ 358,960
Significant Other Observable Inputs (Level 2) | States, municipalities, and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	2,760,981	2,786,804
Significant Other Observable Inputs (Level 2) | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	28,381	28,031
Significant Other Observable Inputs (Level 2) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	13,781,594	13,441,196
Significant Other Observable Inputs (Level 2) | Corporates | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	7,204,316	6,998,900
Significant Other Observable Inputs (Level 2) | Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	4,632,305	4,551,737
Significant Other Observable Inputs (Level 2) | Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	1,944,973	1,890,559
Significant Other Observable Inputs (Level 2) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Significant Other Observable Inputs (Level 2) | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	66,598	65,907
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 491,434	$ 474,114
Percentage of total	3.00%	3.00%	3.00%
Significant Unobservable Inputs (Level 3) | U.S. Government direct, guaranteed, and government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 0	$ 0
Significant Unobservable Inputs (Level 3) | States, municipalities, and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Significant Unobservable Inputs (Level 3) | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Significant Unobservable Inputs (Level 3) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 426,186	420,065
Percentage of total	3.00%		3.00%
Significant Unobservable Inputs (Level 3) | Corporates | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 180,934	183,356
Significant Unobservable Inputs (Level 3) | Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	129,897	124,193
Significant Unobservable Inputs (Level 3) | Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	115,355	112,516
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 41,655	42,866
Percentage of total	0.00%		0.00%
Significant Unobservable Inputs (Level 3) | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 23,593	$ 11,183