Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 16,969,325	$ 15,245,861
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	$ 407,446	$ 372,798
States, municipalities, and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,219,715	1,410,128
Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	22,018	23,077
Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	14,671,836	12,882,480
Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	71,581	63,503
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	149,355	56,694
Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	427,374	437,181
Financial | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,735,095	3,202,736
Financial | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	396,786	408,027
Utilities | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,323,845	2,113,043
Utilities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	30,588	29,154
Energy | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,820,097	1,626,091
Other corporate sectors | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,792,799	5,940,610
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	$ 44	$ 45,302
Percentage of total	0.00%	0.30%
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	44	45,302
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) | 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
Quoted Prices in Active Markets for Identical Assets (Level 1) | Redeemable preferred stocks
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) | Financial | 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) | Financial | Redeemable preferred stocks
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) | Utilities | 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) | Utilities | Redeemable preferred stocks
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) | Energy | 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) | Other corporate sectors | Corporates
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	$ 16,300,841	$ 14,577,456
Percentage of total	96.10%	95.60%
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	$ 407,446	$ 372,798
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	1,219,671	1,364,826
Significant Other Observable Inputs (Level 2) | Foreign governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	22,018	23,077
Significant Other Observable Inputs (Level 2) | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	14,089,026	12,322,880
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	135,306	56,694
Significant Other Observable Inputs (Level 2) | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	427,374	437,181
Significant Other Observable Inputs (Level 2) | Financial | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,673,089	3,141,611
Significant Other Observable Inputs (Level 2) | Financial | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	396,786	408,027
Significant Other Observable Inputs (Level 2) | Utilities | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,168,115	1,959,143
Significant Other Observable Inputs (Level 2) | Utilities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	30,588	29,154
Significant Other Observable Inputs (Level 2) | Energy | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,779,281	1,598,976
Significant Other Observable Inputs (Level 2) | Other corporate sectors | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,468,541	5,623,150
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 668,440	$ 623,103
Percentage of total	3.90%	4.10%
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	582,810	559,600
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	71,581	63,503
Significant Unobservable Inputs (Level 3) | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	14,049	0
Significant Unobservable Inputs (Level 3) | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Significant Unobservable Inputs (Level 3) | Financial | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	62,006	61,125
Significant Unobservable Inputs (Level 3) | Financial | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Significant Unobservable Inputs (Level 3) | Utilities | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	155,730	153,900
Significant Unobservable Inputs (Level 3) | Utilities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Significant Unobservable Inputs (Level 3) | Energy | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	40,816	27,115
Significant Unobservable Inputs (Level 3) | Other corporate sectors | Corporates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 324,258	$ 317,460