Summary of Aggregate Fair Value and Gross Unrealized Loss by the Length of Time those Securities had Continuously been in a Gross Unrealized Loss Position as Well as the Number of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011 Investment	Dec. 31, 2010 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	239,000	313,000
Fair Value	$ 424,288	$ 757,305
Gross Unrealized Loss	8,288	20,762
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	20,000	36,000
Fair Value	50,337	163,253
Gross Unrealized Loss	610	4,499
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	4,000	36,000
Fair Value	7,714	163,253
Gross Unrealized Loss	16	4,499
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds | 7-12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	16,000
Fair Value	42,623
Gross Unrealized Loss	594
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	27,000	62,000
Fair Value	33,654	114,612
Gross Unrealized Loss	962	3,805
Fixed maturities | States, municipalities and political subdivisions | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	4,000	57,000
Fair Value	11,970	112,291
Gross Unrealized Loss	42	3,749
Fixed maturities | States, municipalities and political subdivisions | 7-12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	15,000	1,000
Fair Value	19,445	1,004
Gross Unrealized Loss	870	20
Fixed maturities | States, municipalities and political subdivisions | > 12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	8,000	4,000
Fair Value	2,239	1,317
Gross Unrealized Loss	50	36
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	21,000	36,000
Fair Value	73,679	139,226
Gross Unrealized Loss	864	2,434
Fixed maturities | Agency mortgage-backed securities | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	5,000	36,000
Fair Value	31,727	139,226
Gross Unrealized Loss	89	2,434
Fixed maturities | Agency mortgage-backed securities | 7-12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	16,000
Fair Value	41,952
Gross Unrealized Loss	775
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	57,000	55,000
Fair Value	23,948	19,154
Gross Unrealized Loss	2,266	2,393
Fixed maturities | Residential Mortgage Backed Securities | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	9,000	3,000
Fair Value	10,986	3,215
Gross Unrealized Loss	119	20
Fixed maturities | Residential Mortgage Backed Securities | 7-12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	1,000
Fair Value	183
Gross Unrealized Loss	15
Fixed maturities | Residential Mortgage Backed Securities | > 12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	47,000	52,000
Fair Value	12,779	15,939
Gross Unrealized Loss	2,132	2,373
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	9,000	8,000
Fair Value	18,227	28,177
Gross Unrealized Loss	77	292
Fixed maturities | Asset-backed securities | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	8,000	7,000
Fair Value	18,225	28,175
Gross Unrealized Loss	77	292
Fixed maturities | Asset-backed securities | > 12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	1,000	1,000
Fair Value	2	2
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	20,000	18,000
Fair Value	62,682	78,703
Gross Unrealized Loss	800	1,794
Fixed maturities | Commercial mortgage-backed securities | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	14,000	16,000
Fair Value	50,628	78,212
Gross Unrealized Loss	541	1,755
Fixed maturities | Commercial mortgage-backed securities | 7-12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	5,000
Fair Value	11,832
Gross Unrealized Loss	240
Fixed maturities | Commercial mortgage-backed securities | > 12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	1,000	2,000
Fair Value	222	491
Gross Unrealized Loss	19	39
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	85,000	98,000
Fair Value	161,761	214,180
Gross Unrealized Loss	2,709	5,545
Fixed maturities | Corporate bonds | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	36,000	98,000
Fair Value	60,661	214,180
Gross Unrealized Loss	667	5,545
Fixed maturities | Corporate bonds | 7-12 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	49,000
Fair Value	101,100
Gross Unrealized Loss	2,042
Equity Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	7,000	1,000
Fair Value	4,743	322
Gross Unrealized Loss	257	10
Equity Securities | 0-6 Months
Gain (Loss) on Investments [Line Items]
Number of Securities	7,000	1,000
Fair Value	4,743	322
Gross Unrealized Loss	$ 257	$ 10