Investments Including Other-Than-Temporarily Impaired Securities Recognized within Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,746,822	$ 2,729,735
Gross Unrealized Gains	87,868	83,078
Gross Unrealized Losses	(13,507)	(11,672)
Amortized Cost	2,672,461	2,658,329
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,345,225	2,365,934
Gross Unrealized Gains	61,735	52,322
Gross Unrealized Losses	(11,273)	(10,347)
Amortized Cost	2,294,763	2,323,959
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	310,389	397,923
Gross Unrealized Gains	4,858	3,431
Gross Unrealized Losses	(8,732)	(5,965)
Amortized Cost	314,263	400,457
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	547,664	541,007
Gross Unrealized Gains	19,989	19,204
Gross Unrealized Losses	(321)	(558)
Amortized Cost	527,996	522,361
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	364,468	364,622
Gross Unrealized Gains	9,975	8,476
Gross Unrealized Losses	(589)	(998)
Amortized Cost	355,082	357,144
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	33,116	34,087
Gross Unrealized Gains	1,079	1,153
Gross Unrealized Losses	(117)	(138)
Amortized Cost	32,154	33,072
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	216,475	206,413
Gross Unrealized Gains	814	380
Gross Unrealized Losses	(554)	(964)
Amortized Cost	216,215	206,997
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	231,703	206,318
Gross Unrealized Gains	7,789	6,630
Gross Unrealized Losses	(28)	(98)
Amortized Cost	223,942	199,786
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	845,762	811,440
Gross Unrealized Gains	19,657	16,639
Gross Unrealized Losses	(1,288)	(2,198)
Amortized Cost	827,393	796,999
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	641,410	615,564
Gross Unrealized Gains	17,231	13,048
Gross Unrealized Losses	(932)	(1,626)
Amortized Cost	625,111	604,142
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments	159,044	184,295
Gross Unrealized Gains	26,113	30,756
Gross Unrealized Losses	(1,037)	(1,304)
Amortized Cost	133,968	154,843
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments	242,553	179,506
Gross Unrealized Gains	20
Gross Unrealized Losses	(1,197)	(21)
Amortized Cost	$ 243,730	$ 179,527