Investments - Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 13,758,024	$ 14,493,060	[1]
Equity securities available for sale	1,635	1,477	[1]
Total fixed maturities and equity securities	$ 13,759,659	$ 14,494,537
Percentage of total	100.00%	100.00%
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 33,078	$ 112,325
Percentage of total	0.20%	0.80%
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities and equity securities	$ 13,124,523	$ 13,805,433
Percentage of total	95.40%	95.20%
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 602,058
Total fixed maturities and equity securities	$ 602,058	$ 576,779
Percentage of total	4.40%	4.00%
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 13,758,024	$ 14,493,060
Fixed Maturities [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	355,044	369,841
Fixed Maturities [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,426,004	1,454,763
Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	22,800	27,173
Fixed Maturities [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11,411,178	12,005,952
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	70,382	63,232
Fixed Maturities [Member] | Other Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	19,631	22,528
Fixed Maturities [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	452,985	549,571
Fixed Maturities [Member] | Financial [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,007,295	3,066,433
Fixed Maturities [Member] | Financial [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	423,662	520,127
Fixed Maturities [Member] | Utilities [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,176,509	2,529,450
Fixed Maturities [Member] | Utilities [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	29,323	29,444
Fixed Maturities [Member] | Energy [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,403,067	1,663,683
Fixed Maturities [Member] | Other Corporate Sectors [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,824,307	4,746,386
Fixed Maturities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	32,313	111,681
Fixed Maturities [Member] | Level 1 [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 1 [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	1,504
Fixed Maturities [Member] | Level 1 [Member] | Foreign Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 1 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	22,189	86,571
Fixed Maturities [Member] | Level 1 [Member] | Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 1 [Member] | Other Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	661
Fixed Maturities [Member] | Level 1 [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,124	22,945
Fixed Maturities [Member] | Level 1 [Member] | Financial [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	56,517
Fixed Maturities [Member] | Level 1 [Member] | Financial [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,124	17,811
Fixed Maturities [Member] | Level 1 [Member] | Utilities [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	22,189	30,054
Fixed Maturities [Member] | Level 1 [Member] | Utilities [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	5,134
Fixed Maturities [Member] | Level 1 [Member] | Energy [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 1 [Member] | Other Corporate Sectors [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	13,124,523	13,805,433
Fixed Maturities [Member] | Level 2 [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	355,044	369,841
Fixed Maturities [Member] | Level 2 [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,426,004	1,453,259
Fixed Maturities [Member] | Level 2 [Member] | Foreign Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	22,800	27,173
Fixed Maturities [Member] | Level 2 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,858,183	11,406,667
Fixed Maturities [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 2 [Member] | Other Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	19,631	21,867
Fixed Maturities [Member] | Level 2 [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	442,861	526,626
Fixed Maturities [Member] | Level 2 [Member] | Financial [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,945,048	2,940,267
Fixed Maturities [Member] | Level 2 [Member] | Financial [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	413,538	502,316
Fixed Maturities [Member] | Level 2 [Member] | Utilities [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,020,268	2,366,408
Fixed Maturities [Member] | Level 2 [Member] | Utilities [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	29,323	24,310
Fixed Maturities [Member] | Level 2 [Member] | Energy [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,377,861	1,636,653
Fixed Maturities [Member] | Level 2 [Member] | Other Corporate Sectors [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,515,006	4,463,339
Fixed Maturities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	601,188	575,946
Fixed Maturities [Member] | Level 3 [Member] | U.S. Government Direct, Guaranteed, and Government-Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 3 [Member] | States, Municipalities, and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 3 [Member] | Foreign Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 3 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	530,806	512,714
Fixed Maturities [Member] | Level 3 [Member] | Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	70,382	63,232
Fixed Maturities [Member] | Level 3 [Member] | Other Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 3 [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 3 [Member] | Financial [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	62,247	69,649
Fixed Maturities [Member] | Level 3 [Member] | Financial [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 3 [Member] | Utilities [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	134,052	132,988
Fixed Maturities [Member] | Level 3 [Member] | Utilities [Member] | Redeemable Preferred Stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fixed Maturities [Member] | Level 3 [Member] | Energy [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	25,206	27,030
Fixed Maturities [Member] | Level 3 [Member] | Other Corporate Sectors [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	309,301	283,047
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	1,635	1,477
Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	765	644
Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	0	0
Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	$ 870	$ 833

[1] Certain balances were adjusted to give effect to discontinued operations as described in Note 1—Significant Accounting Policies.