Summary of Aggregate Fair Value and Gross Unrealized Loss by Length of Time Securities had Continuously been in Gross Unrealized Loss Position as Well as Number of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012 Investment	Dec. 31, 2011 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	89	183
Fair Value	$ 91,169	$ 305,121
Gross Unrealized Loss	1,237	10,782
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	9	9
Fair Value	26,062	65,470
Gross Unrealized Loss	55	383
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	7
Fair Value	16,736	58,587
Gross Unrealized Loss	34	98
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4
Fair Value	4,682
Gross Unrealized Loss	3
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	2
Fair Value	4,644	6,883
Gross Unrealized Loss	18	285
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	6	13
Fair Value	4,171	8,530
Gross Unrealized Loss	66	108
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	7
Fair Value	401	5,894
Gross Unrealized Loss	2	72
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	1
Fair Value	1,473	216
Gross Unrealized Loss	40	1
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	5
Fair Value	2,297	2,420
Gross Unrealized Loss	24	35
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	5	3
Fair Value	18,901	5,087
Gross Unrealized Loss	56	3
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	3
Fair Value	18,901	5,087
Gross Unrealized Loss	56	3
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	43	60
Fair Value	10,906	22,671
Gross Unrealized Loss	837	2,848
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	6
Fair Value	550	6,672
Gross Unrealized Loss	19	184
Fixed maturities | Residential Mortgage Backed Securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		7
Fair Value		5,250
Gross Unrealized Loss		313
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	41	47
Fair Value	10,356	10,749
Gross Unrealized Loss	818	2,351
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	2	8
Fair Value	2,504	11,580
Gross Unrealized Loss	72	75
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities		2
Fair Value		4,933
Gross Unrealized Loss		12
Fixed maturities | Asset-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		5
Fair Value		6,645
Gross Unrealized Loss		63
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	1
Fair Value	2,504	2
Gross Unrealized Loss	72
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	13	12
Fair Value	7,492	13,808
Gross Unrealized Loss	50	593
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	6
Fair Value	2,772	5,465
Gross Unrealized Loss	8	29
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	3
Fair Value	3,777	6,840
Gross Unrealized Loss	36	550
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	3
Fair Value	943	1,503
Gross Unrealized Loss	6	14
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	11	78
Fair Value	21,133	177,975
Gross Unrealized Loss	101	6,772
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	52
Fair Value	6,357	135,516
Gross Unrealized Loss	51	4,539
Fixed maturities | Corporate bonds | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		18
Fair Value		27,561
Gross Unrealized Loss		1,457
Fixed maturities | Corporate bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	8	8
Fair Value	14,776	14,898
Gross Unrealized Loss	50	776
Common Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	3	5
Fair Value	4,423	4,949
Gross Unrealized Loss	278	958
Common Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	4
Fair Value	2,144	3,320
Gross Unrealized Loss	4	587
Common Stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	1
Fair Value	1,789	1,629
Gross Unrealized Loss	259	371
Common Stock | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1
Fair Value	490
Gross Unrealized Loss	$ 15