Summary of Consolidated Investments-Other Than Investments in Related Parties (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Summary of Investments, Other than Investments in Related Parties, Reportable Data [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
Common Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	101,297
Gross Unrealized Gains	16,919
Gross Unrealized Losses	(626)
Amortized Cost	85,004
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	153,788	122,220
Amortized Cost	153,788	122,220
Cash
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	45,336	127,360
Amortized Cost	$ 45,336	$ 127,360