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	Sep. 30, 2011 Investment	Dec. 31, 2010 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	221	313
Fair Value	$ 402,244	$ 757,305
Gross Unrealized Loss	11,927	20,762
Fixed maturities | U.S. Government Treasury bonds, agency bonds and foreign government bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	10	36
Fair Value	87,756	163,253
Gross Unrealized Loss	487	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	8	36
Fair Value	80,840	163,253
Gross Unrealized Loss	230	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	2
Fair Value	6,916
Gross Unrealized Loss	257
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	16	62
Fair Value	18,138	114,612
Gross Unrealized Loss	195	3,805
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7	57
Fair Value	6,133	112,291
Gross Unrealized Loss	26	3,749
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	1
Fair Value	8,809	1,004
Gross Unrealized Loss	131	20
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	4
Fair Value	3,196	1,317
Gross Unrealized Loss	38	36
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	4	36
Fair Value	8,049	139,226
Gross Unrealized Loss	20	2,434
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	36
Fair Value	8,049	139,226
Gross Unrealized Loss	20	2,434
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	57	55
Fair Value	22,643	19,154
Gross Unrealized Loss	2,232	2,393
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7	3
Fair Value	9,407	3,215
Gross Unrealized Loss	124	20
Fixed maturities | Residential Mortgage Backed Securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3
Fair Value	1,520
Gross Unrealized Loss	111
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	47	52
Fair Value	11,716	15,939
Gross Unrealized Loss	1,997	2,373
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	8	8
Fair Value	14,218	28,177
Gross Unrealized Loss	86	292
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7	7
Fair Value	14,216	28,175
Gross Unrealized Loss	86	292
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	1
Fair Value	2	2
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	30	18
Fair Value	60,413	78,703
Gross Unrealized Loss	909	1,794
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	23	16
Fair Value	53,115	78,212
Gross Unrealized Loss	772	1,755
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6
Fair Value	7,078
Gross Unrealized Loss	120
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	2
Fair Value	220	491
Gross Unrealized Loss	17	39
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	96	98
Fair Value	191,027	214,180
Gross Unrealized Loss	7,998	5,545
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	81	98
Fair Value	168,290	214,180
Gross Unrealized Loss	6,819	5,545
Fixed maturities | Corporate bonds | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	15
Fair Value	22,737
Gross Unrealized Loss	1,179
Equity Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	25	1
Fair Value	59,039	322
Gross Unrealized Loss	4,419	10
Equity Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	25	1
Fair Value	59,039	322
Gross Unrealized Loss	$ 4,419	$ 10