Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,140,621	$ 2,223,130
Total investments and cash	2,424,121	2,422,254
Gross Unrealized Gains	78,341	105,248
Gross Unrealized Losses	(27,200)	(1,887)
Amortized Cost	2,372,980	2,318,893
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	1,981,731	2,121,833
Gross Unrealized Gains	48,793	88,329
Gross Unrealized Losses	(25,496)	(1,261)
Amortized Cost	1,958,434	2,034,765
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments and cash	445,969	649,692
Gross Unrealized Gains	3,683	8,654
Gross Unrealized Losses	(5,694)	(36)
Amortized Cost	447,980	641,074
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	445,281	322,947
Gross Unrealized Gains	9,752	18,712
Gross Unrealized Losses	(12,240)	(380)
Amortized Cost	447,769	304,615
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	300,347	384,445
Gross Unrealized Gains	7,966	13,652
Gross Unrealized Losses	(3,925)	(204)
Amortized Cost	296,306	370,997
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	35,277	38,692
Gross Unrealized Gains	1,107	1,053
Gross Unrealized Losses	(224)	(549)
Amortized Cost	34,394	38,188
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	88,714	50,382
Gross Unrealized Gains	795	1,133
Gross Unrealized Losses	(190)	(49)
Amortized Cost	88,109	49,298
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	173,069	204,821
Gross Unrealized Gains	8,386	17,996
Gross Unrealized Losses	(365)	(18)
Amortized Cost	165,048	186,843
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	597,407	678,340
Gross Unrealized Gains	18,254	33,834
Gross Unrealized Losses	(4,704)	(820)
Amortized Cost	583,857	645,326
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	493,074	470,854
Gross Unrealized Gains	17,104	27,129
Gross Unrealized Losses	(2,858)	(25)
Amortized Cost	478,828	443,750
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments and cash	158,890	101,297
Gross Unrealized Gains	29,548	16,919
Gross Unrealized Losses	(1,704)	(626)
Amortized Cost	131,046	85,004
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	152,987	153,788
Amortized Cost	152,987	153,788
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	130,513	45,336
Amortized Cost	$ 130,513	$ 45,336