Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 14,121,462	$ 14,493,060
Equity securities	1,545	$ 1,477
Total fixed maturities and equity securities	$ 14,123,007
Percent of total	100.00%
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 14,121,462
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	1,545
U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	354,556
States, Municipalities, and Political Subdivisions [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,429,363
Foreign Governments [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	24,083
Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	11,697,691
Collateralized Debt Obligations [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	73,668
Other Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	20,632
Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	521,469
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 41,112
Percent of total	0.30%
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 40,400
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	712
Fair Value, Inputs, Level 1 [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | States, Municipalities, and Political Subdivisions [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Foreign Governments [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	25,174
Fair Value, Inputs, Level 1 [Member] | Collateralized Debt Obligations [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Other Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	15,226
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 13,485,365
Percent of total	95.50%
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 13,485,365
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0
Fair Value, Inputs, Level 2 [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	354,556
Fair Value, Inputs, Level 2 [Member] | States, Municipalities, and Political Subdivisions [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,429,363
Fair Value, Inputs, Level 2 [Member] | Foreign Governments [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	24,083
Fair Value, Inputs, Level 2 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	11,150,488
Fair Value, Inputs, Level 2 [Member] | Collateralized Debt Obligations [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 2 [Member] | Other Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	20,632
Fair Value, Inputs, Level 2 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	506,243
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 596,530
Percent of total	4.20%
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 595,697
Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	833
Fair Value, Inputs, Level 3 [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | States, Municipalities, and Political Subdivisions [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Foreign Governments [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	522,029
Fair Value, Inputs, Level 3 [Member] | Collateralized Debt Obligations [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	73,668
Fair Value, Inputs, Level 3 [Member] | Other Asset-backed Securities [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Financial [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,952,069
Financial [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	492,074
Financial [Member] | Fair Value, Inputs, Level 1 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Financial [Member] | Fair Value, Inputs, Level 1 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	15,226
Financial [Member] | Fair Value, Inputs, Level 2 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,883,531
Financial [Member] | Fair Value, Inputs, Level 2 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	476,848
Financial [Member] | Fair Value, Inputs, Level 3 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	68,538
Financial [Member] | Fair Value, Inputs, Level 3 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Utilities [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,369,965
Utilities [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	29,395
Utilities [Member] | Fair Value, Inputs, Level 1 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	25,174
Utilities [Member] | Fair Value, Inputs, Level 1 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Utilities [Member] | Fair Value, Inputs, Level 2 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,209,586
Utilities [Member] | Fair Value, Inputs, Level 2 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	29,395
Utilities [Member] | Fair Value, Inputs, Level 3 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	135,205
Utilities [Member] | Fair Value, Inputs, Level 3 [Member] | Redeemable Preferred Stocks [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Energy [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,610,631
Energy [Member] | Fair Value, Inputs, Level 1 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Energy [Member] | Fair Value, Inputs, Level 2 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,583,301
Energy [Member] | Fair Value, Inputs, Level 3 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	27,330
Other Corporate Sectors [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	4,765,026
Other Corporate Sectors [Member] | Fair Value, Inputs, Level 1 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Other Corporate Sectors [Member] | Fair Value, Inputs, Level 2 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	4,474,070
Other Corporate Sectors [Member] | Fair Value, Inputs, Level 3 [Member] | Corporates [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 290,956