Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 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	$ 14,080,199	$ 14,493,060
Equity securities	1,582	$ 1,477
Total fixed maturities and equity securities	$ 14,081,781
Percent of total	100.00%
Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 14,080,199
Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	1,582
U.S. Government direct, guaranteed, and government-sponsored enterprises [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	369,757
States, municipalities, and political subdivisions [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,433,677
Foreign governments [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	22,446
Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	11,705,212
Collateralized debt obligations [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	72,112
Other asset-backed securities [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	20,251
Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	456,744
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 25,971
Percent of total	0.20%
Level 1 [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 25,248
Level 1 [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	723
Level 1 [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 1 [Member] | States, municipalities, and political subdivisions [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 1 [Member] | Foreign governments [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 1 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	15,180
Level 1 [Member] | Collateralized debt obligations [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 1 [Member] | Other asset-backed securities [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 1 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	10,068
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 13,443,169
Percent of total	95.50%
Level 2 [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 13,443,169
Level 2 [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0
Level 2 [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	369,757
Level 2 [Member] | States, municipalities, and political subdivisions [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,433,677
Level 2 [Member] | Foreign governments [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	22,446
Level 2 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	11,150,362
Level 2 [Member] | Collateralized debt obligations [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 2 [Member] | Other asset-backed securities [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	20,251
Level 2 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	446,676
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 612,641
Percent of total	4.30%
Level 3 [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 611,782
Level 3 [Member] | Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	859
Level 3 [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 3 [Member] | States, municipalities, and political subdivisions [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 3 [Member] | Foreign governments [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 3 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	539,670
Level 3 [Member] | Collateralized debt obligations [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	72,112
Level 3 [Member] | Other asset-backed securities [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Level 3 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [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 available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,984,078
Financial [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	426,861
Financial [Member] | Level 1 [Member] | Corporates [Member] | Fixed maturities available for sale [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] | Level 1 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	10,068
Financial [Member] | Level 2 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,920,889
Financial [Member] | Level 2 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	416,793
Financial [Member] | Level 3 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	63,189
Financial [Member] | Level 3 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [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 available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,358,166
Utilities [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	29,883
Utilities [Member] | Level 1 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	15,180
Utilities [Member] | Level 1 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [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] | Level 2 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,206,803
Utilities [Member] | Level 2 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	29,883
Utilities [Member] | Level 3 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	136,183
Utilities [Member] | Level 3 [Member] | Redeemable preferred stocks [Member] | Fixed maturities available for sale [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 available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,515,448
Energy [Member] | Level 1 [Member] | Corporates [Member] | Fixed maturities available for sale [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] | Level 2 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,488,583
Energy [Member] | Level 3 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	26,865
Other corporate sectors [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	4,847,520
Other corporate sectors [Member] | Level 1 [Member] | Corporates [Member] | Fixed maturities available for sale [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] | Level 2 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	4,534,087
Other corporate sectors [Member] | Level 3 [Member] | Corporates [Member] | Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 313,433