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	Dec. 31, 2012 Investment	Dec. 31, 2011 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	99	183
Fair Value	$ 156,071	$ 305,121
Gross Unrealized Loss	1,261	10,782
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign government bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	12	9
Fair Value	42,530	65,470
Gross Unrealized Loss	36	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	8	7
Fair Value	23,760	58,587
Gross Unrealized Loss	22	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	3
Fair Value	14,118
Gross Unrealized Loss	11
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	4,652	6,883
Gross Unrealized Loss	3	285
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	14	13
Fair Value	24,207	8,530
Gross Unrealized Loss	380	108
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	10	7
Fair Value	21,299	5,894
Gross Unrealized Loss	325	72
Fixed maturities | States, municipalities and political subdivisions | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		1
Fair Value		216
Gross Unrealized Loss		1
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	5
Fair Value	2,908	2,420
Gross Unrealized Loss	55	35
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	12	3
Fair Value	64,187	5,087
Gross Unrealized Loss	204	3
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	10	3
Fair Value	62,516	5,087
Gross Unrealized Loss	174	3
Fixed maturities | Agency mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	1,671
Gross Unrealized Loss	30
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	41	60
Fair Value	9,077	22,671
Gross Unrealized Loss	549	2,848
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	6	6
Fair Value	1,825	6,672
Gross Unrealized Loss	22	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	35	47
Fair Value	7,252	10,749
Gross Unrealized Loss	527	2,351
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	2	8
Fair Value	2,369	11,580
Gross Unrealized Loss	49	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	2,369	2
Gross Unrealized Loss	49
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	12	12
Fair Value	3,484	13,808
Gross Unrealized Loss	18	593
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	7	6
Fair Value	2,639	5,465
Gross Unrealized Loss	7	29
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		3
Fair Value		6,840
Gross Unrealized Loss		550
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	3
Fair Value	845	1,503
Gross Unrealized Loss	11	14
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	6	78
Fair Value	10,217	177,975
Gross Unrealized Loss	25	6,772
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2	52
Fair Value	3,528	135,516
Gross Unrealized Loss	6	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	4	8
Fair Value	6,689	14,898
Gross Unrealized Loss	19	776
Common Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	14	5
Fair Value	25,288	4,949
Gross Unrealized Loss	626	958
Common Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	13	4
Fair Value	23,345	3,320
Gross Unrealized Loss	522	587
Common Stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	1
Fair Value	1,943	1,629
Gross Unrealized Loss	$ 104	$ 371