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	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 1,256,887	$ 1,010,970
Less than 12 months Gross Unrealized Losses	(24,493)	(20,517)
Greater than 12 months Fair Value	127,403	156,975
Greater than 12 months Gross Unrealized Losses	(5,706)	(8,881)
Fair Value	1,384,290	1,167,945
Gross Unrealized Losses	(30,199)	(29,398)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	1,111,873	952,439
Less than 12 months Gross Unrealized Losses	(19,074)	(17,504)
Greater than 12 months Fair Value	125,409	156,975
Greater than 12 months Gross Unrealized Losses	(5,340)	(8,881)
Fair Value	1,237,282	1,109,414
Gross Unrealized Losses	(24,414)	(26,385)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	150,891	142,233
Less than 12 months Gross Unrealized Losses	(2,570)	(3,032)
Greater than 12 months Fair Value	16,819	22,230
Greater than 12 months Gross Unrealized Losses	(2,558)	(6,182)
Fair Value	167,710	164,463
Gross Unrealized Losses	(5,128)	(9,214)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	137,731	50,577
Less than 12 months Gross Unrealized Losses	(3,111)	(549)
Greater than 12 months Fair Value	13,255	4,808
Greater than 12 months Gross Unrealized Losses	(925)	(232)
Fair Value	150,986	55,385
Gross Unrealized Losses	(4,036)	(781)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	349,119	164,817
Less than 12 months Gross Unrealized Losses	(6,155)	(1,315)
Greater than 12 months Fair Value	12,401	29,862
Greater than 12 months Gross Unrealized Losses	(430)	(767)
Fair Value	361,520	194,679
Gross Unrealized Losses	(6,585)	(2,082)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	953	3,910
Less than 12 months Gross Unrealized Losses	(18)	(5)
Greater than 12 months Fair Value	926	1,684
Greater than 12 months Gross Unrealized Losses	(37)	(77)
Fair Value	1,879	5,594
Gross Unrealized Losses	(55)	(82)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	95,514	112,479
Less than 12 months Gross Unrealized Losses	(970)	(663)
Greater than 12 months Fair Value	48,093	81,477
Greater than 12 months Gross Unrealized Losses	(208)	(961)
Fair Value	143,607	193,956
Gross Unrealized Losses	(1,178)	(1,624)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	51,932	83,024
Less than 12 months Gross Unrealized Losses	(1,164)	(1,826)
Greater than 12 months Fair Value	7,910	3,065
Greater than 12 months Gross Unrealized Losses	(740)	(38)
Fair Value	59,842	86,089
Gross Unrealized Losses	(1,904)	(1,864)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	497,518	364,230
Less than 12 months Gross Unrealized Losses	(8,307)	(3,809)
Greater than 12 months Fair Value	69,330	116,088
Greater than 12 months Gross Unrealized Losses	(1,415)	(1,843)
Fair Value	566,848	480,318
Gross Unrealized Losses	(9,722)	(5,652)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	325,733	395,399
Less than 12 months Gross Unrealized Losses	(5,086)	(10,114)
Greater than 12 months Fair Value	26,005	13,849
Greater than 12 months Gross Unrealized Losses	(442)	(624)
Fair Value	351,738	409,248
Gross Unrealized Losses	(5,528)	(10,738)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	145,014	58,531
Less than 12 months Gross Unrealized Losses	(5,419)	(3,013)
Greater than 12 months Fair Value	1,994
Greater than 12 months Gross Unrealized Losses	(366)
Fair Value	147,008	58,531
Gross Unrealized Losses	$ (5,785)	$ (3,013)