Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,146,355	$ 2,223,130
Total investments and cash	2,373,041	2,422,254
Gross Unrealized Gains	73,798	105,248
Gross Unrealized Losses	(31,171)	(1,887)
Amortized Cost	2,330,414	2,318,893
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	2,007,806	2,121,833
Gross Unrealized Gains	48,560	88,329
Gross Unrealized Losses	(29,261)	(1,261)
Amortized Cost	1,988,507	2,034,765
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Total investments and cash	464,167	649,692
Gross Unrealized Gains	3,538	8,654
Gross Unrealized Losses	(7,492)	(36)
Amortized Cost	468,121	641,074
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	452,084	322,947
Gross Unrealized Gains	10,281	18,712
Gross Unrealized Losses	(13,114)	(380)
Amortized Cost	454,917	304,615
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	320,150	384,445
Gross Unrealized Gains	7,782	13,652
Gross Unrealized Losses	(4,645)	(204)
Amortized Cost	317,013	370,997
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	37,335	38,692
Gross Unrealized Gains	1,196	1,053
Gross Unrealized Losses	(221)	(549)
Amortized Cost	36,360	38,188
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	75,311	50,382
Gross Unrealized Gains	747	1,133
Gross Unrealized Losses	(113)	(49)
Amortized Cost	74,677	49,298
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	173,388	204,821
Gross Unrealized Gains	8,731	17,996
Gross Unrealized Losses	(514)	(18)
Amortized Cost	165,171	186,843
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	606,184	678,340
Gross Unrealized Gains	18,456	33,834
Gross Unrealized Losses	(5,493)	(820)
Amortized Cost	593,221	645,326
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	485,371	470,854
Gross Unrealized Gains	16,285	27,129
Gross Unrealized Losses	(3,162)	(25)
Amortized Cost	472,248	443,750
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments and cash	138,549	101,297
Gross Unrealized Gains	25,238	16,919
Gross Unrealized Losses	(1,910)	(626)
Amortized Cost	115,221	85,004
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	124,726	153,788
Amortized Cost	124,726	153,788
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	101,960	45,336
Amortized Cost	$ 101,960	$ 45,336