Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Total investments	$ 2,191,827	$ 2,223,130
Total investments and cash	2,574,586	2,422,254
Gross Unrealized Gains	69,554	105,248
Gross Unrealized Losses	(33,530)	(1,887)
Amortized Cost	2,538,562	2,318,893
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	296,250	153,788
Amortized Cost	296,250	153,788
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	86,509	45,336
Amortized Cost	86,509	45,336
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments	2,047,873	2,121,833
Gross Unrealized Gains	43,854	88,329
Gross Unrealized Losses	(32,980)	(1,261)
Amortized Cost	2,036,999	2,034,765
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments	441,685	649,692
Gross Unrealized Gains	2,854	8,654
Gross Unrealized Losses	(8,855)	(36)
Amortized Cost	447,686	641,074
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments	460,422	322,947
Gross Unrealized Gains	9,298	18,712
Gross Unrealized Losses	(13,651)	(380)
Amortized Cost	464,775	304,615
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	301,274	384,445
Gross Unrealized Gains	6,779	13,652
Gross Unrealized Losses	(6,016)	(204)
Amortized Cost	300,511	370,997
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments	41,755	38,692
Gross Unrealized Gains	1,212	1,053
Gross Unrealized Losses	(161)	(549)
Amortized Cost	40,704	38,188
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	125,133	50,382
Gross Unrealized Gains	653	1,133
Gross Unrealized Losses	(480)	(49)
Amortized Cost	124,960	49,298
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	172,750	204,821
Gross Unrealized Gains	7,656	17,996
Gross Unrealized Losses	(374)	(18)
Amortized Cost	165,468	186,843
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments	640,912	678,340
Gross Unrealized Gains	16,300	33,834
Gross Unrealized Losses	(7,031)	(820)
Amortized Cost	631,643	645,326
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments	504,854	470,854
Gross Unrealized Gains	15,402	27,129
Gross Unrealized Losses	(3,443)	(25)
Amortized Cost	492,895	443,750
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments	143,954	101,297
Gross Unrealized Gains	25,700	16,919
Gross Unrealized Losses	(550)	(626)
Amortized Cost	$ 118,804	$ 85,004