Cash and Investments Including Other-Than-Temporarily Impaired Securities Recognized within Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,422,254	$ 2,233,498
Gross Unrealized Gains	105,248	105,381
Gross Unrealized Losses	(1,887)	(11,740)
Amortized Cost	2,318,893	2,139,857
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	2,121,833	1,888,069
Gross Unrealized Gains	88,329	82,141
Gross Unrealized Losses	(1,261)	(10,782)
Amortized Cost	2,034,765	1,816,710
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	649,692	336,070
Gross Unrealized Gains	8,654	8,979
Gross Unrealized Losses	(36)	(383)
Amortized Cost	641,074	327,474
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	322,947	410,836
Gross Unrealized Gains	18,712	28,887
Gross Unrealized Losses	(380)	(108)
Amortized Cost	304,615	382,057
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	384,445	395,860
Gross Unrealized Gains	13,652	17,321
Gross Unrealized Losses	(204)	(3)
Amortized Cost	370,997	378,542
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	38,692	23,148
Gross Unrealized Gains	1,053	8
Gross Unrealized Losses	(549)	(2,848)
Amortized Cost	38,188	25,988
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	50,382	48,934
Gross Unrealized Gains	1,133	695
Gross Unrealized Losses	(49)	(75)
Amortized Cost	49,298	48,314
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	204,821	216,034
Gross Unrealized Gains	17,996	10,508
Gross Unrealized Losses	(18)	(593)
Amortized Cost	186,843	206,119
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	678,340	683,976
Gross Unrealized Gains	33,834	28,532
Gross Unrealized Losses	(820)	(3,519)
Amortized Cost	645,326	658,963
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	470,854	457,187
Gross Unrealized Gains	27,129	15,743
Gross Unrealized Losses	(25)	(6,772)
Amortized Cost	443,750	448,216
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments and cash	101,297	95,849
Gross Unrealized Gains	16,919	23,240
Gross Unrealized Losses	(626)	(958)
Amortized Cost	85,004	73,567
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	153,788	122,220
Amortized Cost	153,788	122,220
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	45,336	127,360
Amortized Cost	$ 45,336	$ 127,360