Investments - Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 14,128,324	$ 12,879,133	[1]
Equity securities	1,427	1,884	[1]
Total fixed maturities and equity securities	14,129,751
Percent of total	100.00%
Fixed Maturities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	14,128,324
Fixed Maturities [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	398,979
Fixed Maturities [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,409,251
Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	38,972
Fixed Maturities [Member] | Corporates [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	11,637,708
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	66,219
Fixed Maturities [Member] | Other Asset-backed Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	34,134
Fixed Maturities [Member] | Redeemable Preferred Stocks [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	543,061
Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Equity securities	1,427
Fair Value, Inputs, Level 1 [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Total fixed maturities and equity securities	76,083
Percent of total	0.50%
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	75,489
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member] | Corporates [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	52,116
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member] | Other Asset-backed Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 1 [Member] | Fixed Maturities [Member] | Redeemable Preferred Stocks [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	23,373
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Equity securities	594
Fair Value, Inputs, Level 2 [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Total fixed maturities and equity securities	13,577,283
Percent of total	96.10%
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	13,577,283
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	398,979
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,409,251
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	38,972
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member] | Corporates [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	11,176,259
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member] | Other Asset-backed Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	34,134
Fair Value, Inputs, Level 2 [Member] | Fixed Maturities [Member] | Redeemable Preferred Stocks [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	519,688
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Equity securities	0
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Total fixed maturities and equity securities	476,385
Percent of total	3.40%
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	475,552
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member] | Corporates [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	409,333
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	66,219
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member] | Other Asset-backed Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Fixed Maturities [Member] | Redeemable Preferred Stocks [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Equity securities	$ 833

[1] Derived from audited financial statements