Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,905,057	$ 2,729,735
Gross Unrealized Gains	63,004	83,078
Gross Unrealized Losses	(23,988)	(11,672)
Amortized Cost	2,866,041	2,658,329
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,396,987	2,365,934
Gross Unrealized Gains	46,485	52,322
Gross Unrealized Losses	(16,670)	(10,347)
Amortized Cost	2,367,172	2,323,959
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	301,915	397,923
Gross Unrealized Gains	4,327	3,431
Gross Unrealized Losses	(8,812)	(5,965)
Amortized Cost	306,400	400,457
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	571,104	541,007
Gross Unrealized Gains	17,402	19,204
Gross Unrealized Losses	(927)	(558)
Amortized Cost	554,629	522,361
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	360,198	364,622
Gross Unrealized Gains	7,200	8,476
Gross Unrealized Losses	(912)	(998)
Amortized Cost	353,910	357,144
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	31,568	34,087
Gross Unrealized Gains	854	1,153
Gross Unrealized Losses	(104)	(138)
Amortized Cost	30,818	33,072
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	197,934	206,413
Gross Unrealized Gains	591	380
Gross Unrealized Losses	(846)	(964)
Amortized Cost	198,189	206,997
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	204,876	206,318
Gross Unrealized Gains	5,335	6,630
Gross Unrealized Losses	(958)	(98)
Amortized Cost	200,499	199,786
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	794,576	811,440
Gross Unrealized Gains	13,980	16,639
Gross Unrealized Losses	(2,820)	(2,198)
Amortized Cost	783,416	796,999
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	729,392	615,564
Gross Unrealized Gains	10,776	13,048
Gross Unrealized Losses	(4,111)	(1,626)
Amortized Cost	722,727	604,142
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	274,928	184,295
Gross Unrealized Gains	16,486	30,756
Gross Unrealized Losses	(7,318)	(1,304)
Amortized Cost	265,760	154,843
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	233,142	179,506
Gross Unrealized Gains	33
Gross Unrealized Losses		(21)
Amortized Cost	$ 233,109	$ 179,527