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	Jun. 30, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 223,814	$ 1,010,970
Less than 12 months Gross Unrealized Losses	(3,428)	(20,517)
Greater than 12 months Fair Value	178,154	156,975
Greater than 12 months Gross Unrealized Losses	(7,069)	(8,881)
Fair Value	401,968	1,167,945
Gross Unrealized Losses	(10,497)	(29,398)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	204,644	952,439
Less than 12 months Gross Unrealized Losses	(1,536)	(17,504)
Greater than 12 months Fair Value	174,304	156,975
Greater than 12 months Gross Unrealized Losses	(6,739)	(8,881)
Fair Value	378,948	1,109,414
Gross Unrealized Losses	(8,275)	(26,385)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	21,279	142,233
Less than 12 months Gross Unrealized Losses	(385)	(3,032)
Greater than 12 months Fair Value	23,935	22,230
Greater than 12 months Gross Unrealized Losses	(3,475)	(6,182)
Fair Value	45,214	164,463
Gross Unrealized Losses	(3,860)	(9,214)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	22,959	50,577
Less than 12 months Gross Unrealized Losses	(325)	(549)
Greater than 12 months Fair Value	9,905	4,808
Greater than 12 months Gross Unrealized Losses	(277)	(232)
Fair Value	32,864	55,385
Gross Unrealized Losses	(602)	(781)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	33,608	164,817
Less than 12 months Gross Unrealized Losses	(55)	(1,315)
Greater than 12 months Fair Value	15,636	29,862
Greater than 12 months Gross Unrealized Losses	(128)	(767)
Fair Value	49,244	194,679
Gross Unrealized Losses	(183)	(2,082)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	1,407	3,910
Less than 12 months Gross Unrealized Losses	(29)	(5)
Greater than 12 months Fair Value	1,737	1,684
Greater than 12 months Gross Unrealized Losses	(101)	(77)
Fair Value	3,144	5,594
Gross Unrealized Losses	(130)	(82)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	61,183	112,479
Less than 12 months Gross Unrealized Losses	(169)	(663)
Greater than 12 months Fair Value	79,694	81,477
Greater than 12 months Gross Unrealized Losses	(667)	(961)
Fair Value	140,877	193,956
Gross Unrealized Losses	(836)	(1,624)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	6,584	83,024
Less than 12 months Gross Unrealized Losses	(19)	(1,826)
Greater than 12 months Fair Value	13,722	3,065
Greater than 12 months Gross Unrealized Losses	(1,009)	(38)
Fair Value	20,306	86,089
Gross Unrealized Losses	(1,028)	(1,864)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	102,782	364,230
Less than 12 months Gross Unrealized Losses	(272)	(3,809)
Greater than 12 months Fair Value	110,789	116,088
Greater than 12 months Gross Unrealized Losses	(1,905)	(1,843)
Fair Value	213,571	480,318
Gross Unrealized Losses	(2,177)	(5,652)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	57,624	395,399
Less than 12 months Gross Unrealized Losses	(554)	(10,114)
Greater than 12 months Fair Value	29,675	13,849
Greater than 12 months Gross Unrealized Losses	(1,082)	(624)
Fair Value	87,299	409,248
Gross Unrealized Losses	(1,636)	(10,738)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	19,170	58,531
Less than 12 months Gross Unrealized Losses	(1,892)	(3,013)
Greater than 12 months Fair Value	3,850
Greater than 12 months Gross Unrealized Losses	(330)
Fair Value	23,020	58,531
Gross Unrealized Losses	$ (2,222)	$ (3,013)