Cash And Investments Including Other-Than-Temporarily Impaired ("OTTI") Securities Recognized within Other Comprehensive Income ("OCI") (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,335,903	$ 2,233,498
Gross Unrealized Gains	121,344	105,381
Gross Unrealized Losses	(3,354)	(11,740)
Amortized Cost	2,217,913	2,139,857
OTTI Recognized in OCI	(887)	(1,682)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	2,024,429	1,888,069
Gross Unrealized Gains	91,635	82,141
Gross Unrealized Losses	(2,837)	(10,782)
Amortized Cost	1,935,631	1,816,710
OTTI Recognized in OCI	(887)	(1,682)
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	454,523	336,070
Gross Unrealized Gains	10,330	8,979
Gross Unrealized Losses	(160)	(383)
Amortized Cost	444,353	327,474
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	426,218	410,836
Gross Unrealized Gains	29,960	28,887
Gross Unrealized Losses	(115)	(108)
Amortized Cost	396,373	382,057
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	398,310	395,860
Gross Unrealized Gains	17,393	17,321
Gross Unrealized Losses	(44)	(3)
Amortized Cost	380,961	378,542
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	37,805	23,148
Gross Unrealized Gains	44	8
Gross Unrealized Losses	(1,651)	(2,848)
Amortized Cost	39,412	25,988
OTTI Recognized in OCI	(887)	(1,682)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	54,416	48,934
Gross Unrealized Gains	933	695
Gross Unrealized Losses	(33)	(75)
Amortized Cost	53,516	48,314
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	209,273	216,034
Gross Unrealized Gains	14,456	10,508
Gross Unrealized Losses	(55)	(593)
Amortized Cost	194,872	206,119
Fixed maturities | Mortgage-backed and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	699,804	683,976
Gross Unrealized Gains	32,826	28,532
Gross Unrealized Losses	(1,783)	(3,519)
Amortized Cost	668,761	658,963
OTTI Recognized in OCI	(887)	(1,682)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	443,884	457,187
Gross Unrealized Gains	18,519	15,743
Gross Unrealized Losses	(779)	(6,772)
Amortized Cost	426,144	448,216
Equity Securities | Common Stock
Gain (Loss) on Investments [Line Items]
Total investments and cash	103,649	95,849
Gross Unrealized Gains	29,709	23,240
Gross Unrealized Losses	(517)	(958)
Amortized Cost	74,457	73,567
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	161,983	122,220
Amortized Cost	161,983	122,220
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	45,842	127,360
Amortized Cost	$ 45,842	$ 127,360