Cash and Investments Including Other-Than-Temporarily Impaired ("OTTI") Securities Recognized within Other Comprehensive Income ("OCI") (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Gain (Loss) on Investments [Line Items]
Total investments and cash	$ 2,167,330	$ 2,154,328
Gross Unrealized Gains	83,099	69,884
Gross Unrealized (Losses)	(8,545)	(20,772)
Cost or Amortized Cost	2,092,776	2,105,216
OTTI Recognized in OCI	(1,384)	(1,646)
Fixed maturities
Gain (Loss) on Investments [Line Items]
Total investments and cash	1,847,221	1,882,245
Gross Unrealized Gains	56,140	47,409
Gross Unrealized (Losses)	(8,288)	(20,762)
Cost or Amortized Cost	1,799,369	1,855,598
OTTI Recognized in OCI	(1,384)	(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	291,492	324,145
Gross Unrealized Gains	5,492	5,229
Gross Unrealized (Losses)	(610)	(4,499)
Cost or Amortized Cost	286,610	323,415
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Total investments and cash	374,873	392,250
Gross Unrealized Gains	15,910	11,903
Gross Unrealized (Losses)	(962)	(3,805)
Cost or Amortized Cost	359,925	384,152
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	365,497	382,628
Gross Unrealized Gains	12,752	10,127
Gross Unrealized (Losses)	(864)	(2,434)
Cost or Amortized Cost	353,609	374,935
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	25,948	20,463
Gross Unrealized Gains	40	24
Gross Unrealized (Losses)	(2,266)	(2,393)
Cost or Amortized Cost	28,174	22,832
OTTI Recognized in OCI	(1,384)	(1,646)
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	52,559	46,093
Gross Unrealized Gains	539	247
Gross Unrealized (Losses)	(77)	(292)
Cost or Amortized Cost	52,097	46,138
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	218,429	190,015
Gross Unrealized Gains	6,480	4,804
Gross Unrealized (Losses)	(800)	(1,794)
Cost or Amortized Cost	212,749	187,005
Fixed maturities | Mortgage- and asset-backed securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	662,433	639,199
Gross Unrealized Gains	19,811	15,202
Gross Unrealized (Losses)	(4,007)	(6,913)
Cost or Amortized Cost	646,629	630,910
OTTI Recognized in OCI	(1,384)	(1,646)
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Total investments and cash	518,423	526,651
Gross Unrealized Gains	14,927	15,075
Gross Unrealized (Losses)	(2,709)	(5,545)
Cost or Amortized Cost	506,205	517,121
Equity Securities
Gain (Loss) on Investments [Line Items]
Total investments and cash	94,737	87,258
Gross Unrealized Gains	26,959	22,475
Gross Unrealized (Losses)	(257)	(10)
Cost or Amortized Cost	68,035	64,793
Cash
Gain (Loss) on Investments [Line Items]
Total investments and cash	40,340	31,768
Cost or Amortized Cost	40,340	31,768
Short-term investments
Gain (Loss) on Investments [Line Items]
Total investments and cash	185,032	153,057
Cost or Amortized Cost	$ 185,032	$ 153,057