Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 18,907,147	$ 16,297,932
Percentage of total		100.00%
Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		$ 73,369
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		148,407
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%
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		$ 15,658,110
Percentage of total		96.00%
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	759,409	$ 639,822
Percentage of total		4.00%
Significant Unobservable Inputs (Level 3) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	107,024
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	74,104
Fixed maturities available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 18,907,147	$ 16,297,932
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	$ 437,520	392,668
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	1,717,656	1,436,660
Fixed maturities available for sale | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	27,551	20,816
Fixed maturities available for sale | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	16,501,797	14,226,012
Fixed maturities available for sale | Corporates | Financial
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,780,806	3,935,128
Fixed maturities available for sale | Corporates | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,352,287	2,182,953
Fixed maturities available for sale | Corporates | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,914,884	1,684,209
Fixed maturities available for sale | Corporates | Other corporate sectors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,453,820	6,423,722
Fixed maturities available for sale | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	74,104	73,369
Fixed maturities available for sale | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	148,519	148,407
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%
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	$ 18,147,738
Percentage of total	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	$ 437,520	392,668
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	1,717,656	1,436,660
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	27,551	20,816
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	15,829,669	13,672,541
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	4,628,875	3,891,728
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	2,195,539	2,032,127
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	1,873,482	1,645,077
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	7,131,773	6,103,609
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	135,342	135,425
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	$ 759,409
Percentage of total	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	672,128	553,471
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	151,931	43,400
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	156,748	150,826
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	41,402	39,132
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	322,047	320,113
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	74,104	73,369
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	$ 13,177	$ 12,982