Summary of Consolidated Investments-Other Than Investments in Related Parties (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	$ 2,233,498	$ 2,154,328
Gross Unrealized Gains	105,381	69,884
Gross Unrealized Losses	(11,740)	(20,772)
Amortized Cost	2,139,857	2,105,216
OTTI Recognized in OCI	(1,682)	(1,646)
Fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	1,888,069	1,882,245
Gross Unrealized Gains	82,141	47,409
Gross Unrealized Losses	(10,782)	(20,762)
Amortized Cost	1,816,710	1,855,598
OTTI Recognized in OCI	(1,682)	(1,646)
Fixed maturities | U.S. Government 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	336,070	324,145
Gross Unrealized Gains	8,979	5,229
Gross Unrealized Losses	(383)	(4,499)
Amortized Cost	327,474	323,415
Fixed maturities | States, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	410,836	392,250
Gross Unrealized Gains	28,887	11,903
Gross Unrealized Losses	(108)	(3,805)
Amortized Cost	382,057	384,152
Fixed maturities | Agency mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	395,860	382,628
Gross Unrealized Gains	17,321	10,127
Gross Unrealized Losses	(3)	(2,434)
Amortized Cost	378,542	374,935
Fixed maturities | Residential Mortgage Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	23,148	20,463
Gross Unrealized Gains	8	24
Gross Unrealized Losses	(2,848)	(2,393)
Amortized Cost	25,988	22,832
OTTI Recognized in OCI	(1,682)	(1,646)
Fixed maturities | Asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	48,934	46,093
Gross Unrealized Gains	695	247
Gross Unrealized Losses	(75)	(292)
Amortized Cost	48,314	46,138
Fixed maturities | Commercial mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	216,034	190,015
Gross Unrealized Gains	10,508	4,804
Gross Unrealized Losses	(593)	(1,794)
Amortized Cost	206,119	187,005
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	683,976	639,199
Gross Unrealized Gains	28,532	15,202
Gross Unrealized Losses	(3,519)	(6,913)
Amortized Cost	658,963	630,910
OTTI Recognized in OCI	(1,682)	(1,646)
Fixed maturities | Corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	457,187	526,651
Gross Unrealized Gains	15,743	15,075
Gross Unrealized Losses	(6,772)	(5,545)
Amortized Cost	448,216	517,121
Common stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	95,849	87,258
Gross Unrealized Gains	23,240	22,475
Gross Unrealized Losses	(958)	(10)
Amortized Cost	73,567	64,793
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	122,220	153,057
Amortized Cost	122,220	153,057
Cash
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	127,360	31,768
Amortized Cost	$ 127,360	$ 31,768