Cash and Investments Including Other-Than-Temporarily Impaired ("OTTI") Securities Recognized within Other Comprehensive Income ("OCI") (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [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
Gain (Loss) on Investments [Line Items]
Total investments and cash	122,220	153,057
Amortized Cost	122,220	153,057
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	127,360	31,768
Amortized Cost	$ 127,360	$ 31,768