Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 17,206,885	$ 16,503,365
Percentage of total	3.00%		3.00%
Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 17,206,885	$ 16,503,365
Percentage of total	100.00%	100.00%
U.S. Government direct, guaranteed, and government-sponsored enterprises | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 362,073	$ 355,498
States, municipalities, and political subdivisions | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	2,508,639	2,309,911
Foreign governments | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 42,520	42,464
Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of total	3.00%		3.00%
Corporates | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 14,167,139	13,666,064
Corporates | Financial | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	4,578,807	4,466,431
Corporates | Utilities | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	1,935,107	1,835,147
Corporates | Energy | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	1,392,905	1,357,312
Corporates | Other corporate sectors | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 6,260,320	6,007,174
Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of total	0.00%		0.00%
Collateralized debt obligations | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 45,502	50,364
Other asset-backed securities | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	81,012	79,064
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities available for sale
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 | Fixed maturities available for sale
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 | Fixed maturities available for sale
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 | Fixed maturities available for sale
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 | Fixed maturities available for sale
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 | Fixed maturities available for sale
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 | Fixed maturities available for sale
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 | Energy | Fixed maturities available for sale
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 | Other corporate sectors | Fixed maturities available for sale
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 | Fixed maturities available for sale
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 | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Significant Other Observable Inputs (Level 2) | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 16,689,744	$ 15,974,918
Percentage of total	97.00%	97.00%
Significant Other Observable Inputs (Level 2) | U.S. Government direct, guaranteed, and government-sponsored enterprises | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 362,073	$ 355,498
Significant Other Observable Inputs (Level 2) | States, municipalities, and political subdivisions | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	2,508,639	2,309,911
Significant Other Observable Inputs (Level 2) | Foreign governments | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	42,520	42,464
Significant Other Observable Inputs (Level 2) | Corporates | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	13,695,500	13,187,981
Significant Other Observable Inputs (Level 2) | Corporates | Financial | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	4,445,091	4,332,495
Significant Other Observable Inputs (Level 2) | Corporates | Utilities | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	1,822,748	1,723,832
Significant Other Observable Inputs (Level 2) | Corporates | Energy | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	1,381,989	1,346,212
Significant Other Observable Inputs (Level 2) | Corporates | Other corporate sectors | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	6,045,672	5,785,442
Significant Other Observable Inputs (Level 2) | Collateralized debt obligations | Fixed maturities available for sale
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 | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	81,012	79,064
Significant Unobservable Inputs (Level 3) | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 517,141	$ 528,447
Percentage of total	3.00%	3.00%
Significant Unobservable Inputs (Level 3) | U.S. Government direct, guaranteed, and government-sponsored enterprises | Fixed maturities available for sale
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 | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Significant Unobservable Inputs (Level 3) | Foreign governments | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	0	0
Significant Unobservable Inputs (Level 3) | Corporates | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	471,639	478,083
Significant Unobservable Inputs (Level 3) | Corporates | Financial | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	133,716	133,936
Significant Unobservable Inputs (Level 3) | Corporates | Utilities | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	112,359	111,315
Significant Unobservable Inputs (Level 3) | Corporates | Energy | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	10,916	11,100
Significant Unobservable Inputs (Level 3) | Corporates | Other corporate sectors | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	214,648	221,732
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	45,502	50,364
Significant Unobservable Inputs (Level 3) | Other asset-backed securities | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 0	$ 0