Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 21,305,287	$ 21,213,509
Percentage of total		100.00%
Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		$ 71,598
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		134,575
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
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		20,414,536
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	705,193	798,973
Significant Unobservable Inputs (Level 3) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	101,808
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	63,505
Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 21,305,287
Percentage of total	100.00%
Fixed maturities available for sale | U.S. Government direct, guaranteed, and government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 447,432	467,831
Fixed maturities available for sale | States, municipalities, and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,489,225	2,131,583
Fixed maturities available for sale | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	55,629	54,650
Fixed maturities available for sale | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	18,141,213	18,353,272
Fixed maturities available for sale | Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,467,165	5,396,795
Fixed maturities available for sale | Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,420,498	2,583,947
Fixed maturities available for sale | Corporates | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,932,989	1,963,738
Fixed maturities available for sale | Corporates | Other corporate sectors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	8,320,561	$ 8,408,792
Fixed maturities available for sale | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	63,505
Fixed maturities available for sale | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	108,283
Fixed maturities available for sale | 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
Percentage of total	0.00%	0.00%
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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
Fixed maturities available for sale | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed maturities available for sale | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Other corporate sectors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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
Fixed maturities available for sale | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 20,600,094
Percentage of total	97.00%	96.00%
Fixed maturities available for sale | 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	$ 447,432	$ 467,831
Fixed maturities available for sale | 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	2,489,225	2,131,583
Fixed maturities available for sale | Significant Other Observable Inputs (Level 2) | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	55,629	54,650
Fixed maturities available for sale | Significant Other Observable Inputs (Level 2) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	17,499,525	17,638,767
Fixed maturities available for sale | Significant Other Observable Inputs (Level 2) | Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,303,547	5,222,066
Fixed maturities available for sale | Significant Other Observable Inputs (Level 2) | Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,266,231	2,400,602
Fixed maturities available for sale | Significant Other Observable Inputs (Level 2) | Corporates | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,919,416	1,925,549
Fixed maturities available for sale | Significant Other Observable Inputs (Level 2) | Corporates | Other corporate sectors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	8,010,331	8,090,550
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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	108,283	$ 121,705
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 705,193
Percentage of total	3.00%	4.00%
Fixed maturities available for sale | 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
Fixed maturities available for sale | 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
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	641,688	714,505
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	163,618	174,729
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	154,267	183,345
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Corporates | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	13,573	38,189
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Corporates | Other corporate sectors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	310,230	318,242
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	63,505	71,598
Fixed maturities available for sale | Significant Unobservable Inputs (Level 3) | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 0	$ 12,870