Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,710,763	$ 2,729,735
Gross Unrealized Gains	57,679	83,078
Gross Unrealized Losses	(19,658)	(11,672)
Amortized Cost	2,672,742	2,658,329
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,313,937	2,365,934
Gross Unrealized Gains	38,886	52,322
Gross Unrealized Losses	(16,086)	(10,347)
Amortized Cost	2,291,137	2,323,959
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	295,687	397,923
Gross Unrealized Gains	3,407	3,431
Gross Unrealized Losses	(6,173)	(5,965)
Amortized Cost	298,453	400,457
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	565,188	541,007
Gross Unrealized Gains	12,511	19,204
Gross Unrealized Losses	(2,531)	(558)
Amortized Cost	555,208	522,361
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	343,916	364,622
Gross Unrealized Gains	6,501	8,476
Gross Unrealized Losses	(2,100)	(998)
Amortized Cost	339,515	357,144
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	32,396	34,087
Gross Unrealized Gains	1,010	1,153
Gross Unrealized Losses	(102)	(138)
Amortized Cost	31,488	33,072
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	195,512	206,413
Gross Unrealized Gains	433	380
Gross Unrealized Losses	(488)	(964)
Amortized Cost	195,567	206,997
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	216,185	206,318
Gross Unrealized Gains	4,814	6,630
Gross Unrealized Losses	(1,481)	(98)
Amortized Cost	212,852	199,786
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	788,009	811,440
Gross Unrealized Gains	12,758	16,639
Gross Unrealized Losses	(4,171)	(2,198)
Amortized Cost	779,422	796,999
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	665,053	615,564
Gross Unrealized Gains	10,210	13,048
Gross Unrealized Losses	(3,211)	(1,626)
Amortized Cost	658,054	604,142
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	195,078	184,295
Gross Unrealized Gains	18,733	30,756
Gross Unrealized Losses	(3,572)	(1,304)
Amortized Cost	179,917	154,843
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	201,748	179,506
Gross Unrealized Gains	60
Gross Unrealized Losses		(21)
Amortized Cost	$ 201,688	$ 179,527