Investments Including OTTI Securities Recognized within AOCI (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,937,226	$ 2,729,735
Gross Unrealized Gains	66,693	83,078
Gross Unrealized Losses	(29,398)	(11,672)
Amortized Cost	2,899,931	2,658,329
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,414,210	2,365,934
Gross Unrealized Gains	40,350	52,322
Gross Unrealized Losses	(26,385)	(10,347)
Amortized Cost	2,400,245	2,323,959
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	252,882	397,923
Gross Unrealized Gains	2,273	3,431
Gross Unrealized Losses	(9,214)	(5,965)
Amortized Cost	259,823	400,457
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	576,859	541,007
Gross Unrealized Gains	21,233	19,204
Gross Unrealized Losses	(781)	(558)
Amortized Cost	556,407	522,361
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	379,269	364,622
Gross Unrealized Gains	5,573	8,476
Gross Unrealized Losses	(2,082)	(998)
Amortized Cost	375,778	357,144
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	30,465	34,087
Gross Unrealized Gains	694	1,153
Gross Unrealized Losses	(82)	(138)
Amortized Cost	29,853	33,072
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	225,012	206,413
Gross Unrealized Gains	85	380
Gross Unrealized Losses	(1,624)	(964)
Amortized Cost	226,551	206,997
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	189,713	206,318
Gross Unrealized Gains	3,119	6,630
Gross Unrealized Losses	(1,864)	(98)
Amortized Cost	188,458	199,786
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	824,459	811,440
Gross Unrealized Gains	9,471	16,639
Gross Unrealized Losses	(5,652)	(2,198)
Amortized Cost	820,640	796,999
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	760,010	615,564
Gross Unrealized Gains	7,373	13,048
Gross Unrealized Losses	(10,738)	(1,626)
Amortized Cost	763,375	604,142
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	305,271	184,295
Gross Unrealized Gains	26,341	30,756
Gross Unrealized Losses	(3,013)	(1,304)
Amortized Cost	281,943	154,843
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	217,745	179,506
Gross Unrealized Gains	2
Gross Unrealized Losses		(21)
Amortized Cost	$ 217,743	$ 179,527