Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,382,536	$ 2,422,254
Gross Unrealized Gains	103,634	105,248
Gross Unrealized Losses	(3,031)	(1,887)
Amortized Cost	2,281,933	2,318,893
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	2,078,003	2,121,833
Gross Unrealized Gains	78,922	88,329
Gross Unrealized Losses	(2,784)	(1,261)
Amortized Cost	2,001,865	2,034,765
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments and cash	531,463	649,692
Gross Unrealized Gains	7,988	8,654
Gross Unrealized Losses	(31)	(36)
Amortized Cost	523,506	641,074
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	449,387	322,947
Gross Unrealized Gains	17,619	18,712
Gross Unrealized Losses	(1,744)	(380)
Amortized Cost	433,512	304,615
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	346,829	384,445
Gross Unrealized Gains	11,842	13,652
Gross Unrealized Losses	(640)	(204)
Amortized Cost	335,627	370,997
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	38,350	38,692
Gross Unrealized Gains	1,223	1,053
Gross Unrealized Losses	(187)	(549)
Amortized Cost	37,314	38,188
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	47,092	50,382
Gross Unrealized Gains	1,110	1,133
Gross Unrealized Losses	(29)	(49)
Amortized Cost	46,011	49,298
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	180,939	204,821
Gross Unrealized Gains	13,170	17,996
Gross Unrealized Losses	(29)	(18)
Amortized Cost	167,798	186,843
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	613,210	678,340
Gross Unrealized Gains	27,345	33,834
Gross Unrealized Losses	(885)	(820)
Amortized Cost	586,750	645,326
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	483,943	470,854
Gross Unrealized Gains	25,970	27,129
Gross Unrealized Losses	(124)	(25)
Amortized Cost	458,097	443,750
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments and cash	116,482	101,297
Gross Unrealized Gains	24,712	16,919
Gross Unrealized Losses	(247)	(626)
Amortized Cost	92,017	85,004
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	129,475	153,788
Amortized Cost	129,475	153,788
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	58,576	45,336
Amortized Cost	$ 58,576	$ 45,336