Cash and Investments Including Other-Than-Temporarily Impaired ("OTTI") Securities Recognized within Other Comprehensive Income ("OCI") (Detail) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,194,251	$ 2,154,328
Gross Unrealized Gains	90,069	69,884
Gross Unrealize Losses	(16,346)	(20,772)
Cost or Amortized Cost	2,120,528	2,105,216
OTTI Recognized in OCI	(1,260)	(1,646)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	1,827,973	1,882,245
Gross Unrealized Gains	71,472	47,409
Gross Unrealize Losses	(11,927)	(20,762)
Cost or Amortized Cost	1,768,428	1,855,598
OTTI Recognized in OCI	(1,260)	(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	301,757	324,145
Gross Unrealized Gains	8,656	5,229
Gross Unrealize Losses	(487)	(4,499)
Cost or Amortized Cost	293,588	323,415
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	386,167	392,250
Gross Unrealized Gains	23,394	11,903
Gross Unrealize Losses	(195)	(3,805)
Cost or Amortized Cost	362,968	384,152
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	377,465	382,628
Gross Unrealized Gains	16,932	10,127
Gross Unrealize Losses	(20)	(2,434)
Cost or Amortized Cost	360,553	374,935
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	24,525	20,463
Gross Unrealized Gains	30	24
Gross Unrealize Losses	(2,232)	(2,393)
Cost or Amortized Cost	26,727	22,832
OTTI Recognized in OCI	(1,260)	(1,646)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	49,915	46,093
Gross Unrealized Gains	779	247
Gross Unrealize Losses	(86)	(292)
Cost or Amortized Cost	49,222	46,138
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	215,033	190,015
Gross Unrealized Gains	6,673	4,804
Gross Unrealize Losses	(909)	(1,794)
Cost or Amortized Cost	209,269	187,005
Fixed maturities | Mortgage- and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	666,938	639,199
Gross Unrealized Gains	24,414	15,202
Gross Unrealize Losses	(3,247)	(6,913)
Cost or Amortized Cost	645,771	630,910
OTTI Recognized in OCI	(1,260)	(1,646)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	473,111	526,651
Gross Unrealized Gains	15,008	15,075
Gross Unrealize Losses	(7,998)	(5,545)
Cost or Amortized Cost	466,101	517,121
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	147,794	87,258
Gross Unrealized Gains	18,597	22,475
Gross Unrealize Losses	(4,419)	(10)
Cost or Amortized Cost	133,616	64,793
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	58,935	31,768
Cost or Amortized Cost	58,935	31,768
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	159,549	153,057
Cost or Amortized Cost	$ 159,549	$ 153,057