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, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	$ 578,417	$ 468,937
Less than 12 months Gross Unrealized Losses	(14,544)	(4,029)
Greater than 12 months Fair Value	157,967	282,337
Greater than 12 months Gross Unrealized Losses	(9,444)	(7,622)
Fair Value	736,384	751,274
Gross Unrealized Losses	(23,988)	(11,651)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	450,170	449,247
Less than 12 months Gross Unrealized Losses	(7,226)	(2,732)
Greater than 12 months Fair Value	157,967	282,099
Greater than 12 months Gross Unrealized Losses	(9,444)	(7,615)
Fair Value	608,137	731,346
Gross Unrealized Losses	(16,670)	(10,347)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	37,766	87,915
Less than 12 months Gross Unrealized Losses	(1,866)	(1,061)
Greater than 12 months Fair Value	28,143	117,683
Greater than 12 months Gross Unrealized Losses	(6,946)	(4,904)
Fair Value	65,909	205,598
Gross Unrealized Losses	(8,812)	(5,965)
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	54,653	16,349
Less than 12 months Gross Unrealized Losses	(540)	(60)
Greater than 12 months Fair Value	4,669	37,340
Greater than 12 months Gross Unrealized Losses	(387)	(498)
Fair Value	59,322	53,689
Gross Unrealized Losses	(927)	(558)
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	52,009	18,881
Less than 12 months Gross Unrealized Losses	(259)	(80)
Greater than 12 months Fair Value	23,259	58,301
Greater than 12 months Gross Unrealized Losses	(653)	(918)
Fair Value	75,268	77,182
Gross Unrealized Losses	(912)	(998)
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	5,196	5,625
Less than 12 months Gross Unrealized Losses	(22)	(50)
Greater than 12 months Fair Value	1,947	1,728
Greater than 12 months Gross Unrealized Losses	(82)	(88)
Fair Value	7,143	7,353
Gross Unrealized Losses	(104)	(138)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	28,823	110,275
Less than 12 months Gross Unrealized Losses	(146)	(539)
Greater than 12 months Fair Value	78,725	34,530
Greater than 12 months Gross Unrealized Losses	(700)	(425)
Fair Value	107,548	144,805
Gross Unrealized Losses	(846)	(964)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	47,297	19,741
Less than 12 months Gross Unrealized Losses	(939)	(71)
Greater than 12 months Fair Value	3,440	1,391
Greater than 12 months Gross Unrealized Losses	(19)	(27)
Fair Value	50,737	21,132
Gross Unrealized Losses	(958)	(98)
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	133,325	154,522
Less than 12 months Gross Unrealized Losses	(1,366)	(740)
Greater than 12 months Fair Value	107,371	95,950
Greater than 12 months Gross Unrealized Losses	(1,454)	(1,458)
Fair Value	240,696	250,472
Gross Unrealized Losses	(2,820)	(2,198)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	224,426	190,461
Less than 12 months Gross Unrealized Losses	(3,454)	(871)
Greater than 12 months Fair Value	17,784	31,126
Greater than 12 months Gross Unrealized Losses	(657)	(755)
Fair Value	242,210	221,587
Gross Unrealized Losses	(4,111)	(1,626)
Equity Securities
Gain (Loss) on Investments [Line Items]
Less than 12 months Fair Value	128,247	19,690
Less than 12 months Gross Unrealized Losses	(7,318)	(1,297)
Greater than 12 months Fair Value		238
Greater than 12 months Gross Unrealized Losses		(7)
Fair Value	128,247	19,928
Gross Unrealized Losses	$ (7,318)	$ (1,304)