Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position as Well as Number of Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 391,806	$ 1,010,970
Less than 12 months Gross Unrealized Losses	(11,789)	(20,517)
Greater than 12 months Fair Value	147,344	156,975
Greater than 12 months Gross Unrealized Losses	(6,174)	(8,881)
Fair Value	539,150	1,167,945
Gross Unrealized Losses	(17,963)	(29,398)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	354,384	952,439
Less than 12 months Gross Unrealized Losses	(10,143)	(17,504)
Greater than 12 months Fair Value	147,344	156,975
Greater than 12 months Gross Unrealized Losses	(6,174)	(8,881)
Fair Value	501,728	1,109,414
Gross Unrealized Losses	(16,317)	(26,385)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	29,155	142,233
Less than 12 months Gross Unrealized Losses	(771)	(3,032)
Greater than 12 months Fair Value	19,451	22,230
Greater than 12 months Gross Unrealized Losses	(3,707)	(6,182)
Fair Value	48,606	164,463
Gross Unrealized Losses	(4,478)	(9,214)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	18,911	50,577
Less than 12 months Gross Unrealized Losses	(266)	(549)
Greater than 12 months Fair Value	6,387	4,808
Greater than 12 months Gross Unrealized Losses	(231)	(232)
Fair Value	25,298	55,385
Gross Unrealized Losses	(497)	(781)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	4,082	164,817
Less than 12 months Gross Unrealized Losses	(4)	(1,315)
Greater than 12 months Fair Value	23,634	29,862
Greater than 12 months Gross Unrealized Losses	(319)	(767)
Fair Value	27,716	194,679
Gross Unrealized Losses	(323)	(2,082)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	4,562	3,910
Less than 12 months Gross Unrealized Losses	(32)	(5)
Greater than 12 months Fair Value	1,793	1,684
Greater than 12 months Gross Unrealized Losses	(99)	(77)
Fair Value	6,355	5,594
Gross Unrealized Losses	(131)	(82)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	91,678	112,479
Less than 12 months Gross Unrealized Losses	(790)	(663)
Greater than 12 months Fair Value	70,768	81,477
Greater than 12 months Gross Unrealized Losses	(1,250)	(961)
Fair Value	162,446	193,956
Gross Unrealized Losses	(2,040)	(1,624)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	49,889	83,024
Less than 12 months Gross Unrealized Losses	(1,690)	(1,826)
Greater than 12 months Fair Value	3,362	3,065
Greater than 12 months Gross Unrealized Losses	(28)	(38)
Fair Value	53,251	86,089
Gross Unrealized Losses	(1,718)	(1,864)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	150,211	364,230
Less than 12 months Gross Unrealized Losses	(2,516)	(3,809)
Greater than 12 months Fair Value	99,557	116,088
Greater than 12 months Gross Unrealized Losses	(1,696)	(1,843)
Fair Value	249,768	480,318
Gross Unrealized Losses	(4,212)	(5,652)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	156,107	395,399
Less than 12 months Gross Unrealized Losses	(6,590)	(10,114)
Greater than 12 months Fair Value	21,949	13,849
Greater than 12 months Gross Unrealized Losses	(540)	(624)
Fair Value	178,056	409,248
Gross Unrealized Losses	(7,130)	(10,738)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	37,422	58,531
Less than 12 months Gross Unrealized Losses	(1,646)	(3,013)
Fair Value	37,422	58,531
Gross Unrealized Losses	$ (1,646)	$ (3,013)