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	Sep. 30, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 397,078	$ 1,010,970
Less than 12 months Gross Unrealized Losses	(3,903)	(20,517)
Greater than 12 months Fair Value	143,136	156,975
Greater than 12 months Gross Unrealized Losses	(5,051)	(8,881)
Fair Value	540,214	1,167,945
Gross Unrealized Losses	(8,954)	(29,398)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	359,028	952,439
Less than 12 months Gross Unrealized Losses	(2,413)	(17,504)
Greater than 12 months Fair Value	138,413	156,975
Greater than 12 months Gross Unrealized Losses	(4,774)	(8,881)
Fair Value	497,441	1,109,414
Gross Unrealized Losses	(7,187)	(26,385)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	91,676	142,233
Less than 12 months Gross Unrealized Losses	(969)	(3,032)
Greater than 12 months Fair Value	19,897	22,230
Greater than 12 months Gross Unrealized Losses	(3,012)	(6,182)
Fair Value	111,573	164,463
Gross Unrealized Losses	(3,981)	(9,214)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	49,881	50,577
Less than 12 months Gross Unrealized Losses	(593)	(549)
Greater than 12 months Fair Value	9,932	4,808
Greater than 12 months Gross Unrealized Losses	(176)	(232)
Fair Value	59,813	55,385
Gross Unrealized Losses	(769)	(781)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	52,754	164,817
Less than 12 months Gross Unrealized Losses	(17)	(1,315)
Greater than 12 months Fair Value	13,547	29,862
Greater than 12 months Gross Unrealized Losses	(88)	(767)
Fair Value	66,301	194,679
Gross Unrealized Losses	(105)	(2,082)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	1,317	3,910
Less than 12 months Gross Unrealized Losses	(16)	(5)
Greater than 12 months Fair Value	1,631	1,684
Greater than 12 months Gross Unrealized Losses	(81)	(77)
Fair Value	2,948	5,594
Gross Unrealized Losses	(97)	(82)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	22,422	112,479
Less than 12 months Gross Unrealized Losses	(158)	(663)
Greater than 12 months Fair Value	61,499	81,477
Greater than 12 months Gross Unrealized Losses	(300)	(961)
Fair Value	83,921	193,956
Gross Unrealized Losses	(458)	(1,624)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	20,864	83,024
Less than 12 months Gross Unrealized Losses	(38)	(1,826)
Greater than 12 months Fair Value	8,854	3,065
Greater than 12 months Gross Unrealized Losses	(385)	(38)
Fair Value	29,718	86,089
Gross Unrealized Losses	(423)	(1,864)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	97,357	364,230
Less than 12 months Gross Unrealized Losses	(229)	(3,809)
Greater than 12 months Fair Value	85,531	116,088
Greater than 12 months Gross Unrealized Losses	(854)	(1,843)
Fair Value	182,888	480,318
Gross Unrealized Losses	(1,083)	(5,652)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	120,114	395,399
Less than 12 months Gross Unrealized Losses	(622)	(10,114)
Greater than 12 months Fair Value	23,053	13,849
Greater than 12 months Gross Unrealized Losses	(732)	(624)
Fair Value	143,167	409,248
Gross Unrealized Losses	(1,354)	(10,738)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	38,050	58,531
Less than 12 months Gross Unrealized Losses	(1,490)	(3,013)
Greater than 12 months Fair Value	4,723
Greater than 12 months Gross Unrealized Losses	(277)
Fair Value	42,773	58,531
Gross Unrealized Losses	$ (1,767)	$ (3,013)