Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 16,503,365	$ 21,305,287
Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 16,503,365	$ 21,305,287
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	$ 355,498	$ 447,432
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	2,309,911	2,489,225
Foreign governments | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	42,464	55,629
Corporates | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	13,666,064	18,141,213
Corporates | Financial | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,466,431	5,467,165
Corporates | Utilities | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,835,147	2,420,498
Corporates | Energy | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,357,312	1,932,989
Corporates | Other corporate sectors | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,007,174	8,320,561
Collateralized debt obligations | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	50,364	63,505
Other asset-backed securities | Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	79,064	108,283
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	$ 15,974,918	$ 20,600,094
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	$ 355,498	$ 447,432
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	2,309,911	2,489,225
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	42,464	55,629
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	13,187,981	17,499,525
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	4,332,495	5,303,547
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	1,723,832	2,266,231
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	1,346,212	1,919,416
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	5,785,442	8,010,331
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	79,064	108,283
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	528,447
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	$ 528,447	$ 705,193
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
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	83,046
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	478,083	641,688
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	133,936	163,618
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	111,315	154,267
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	11,100	13,573
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	221,732	310,230
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	50,364
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	50,364	63,505
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