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	Mar. 31, 2013 Investment	Dec. 31, 2012 Investment
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number of Securities	144	99
Fair Value	$ 256,301	$ 156,071
Gross Unrealized Loss	2,784	1,261
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Gain (Loss) on Investments [Line Items]
Number of Securities	8	12
Fair Value	40,455	42,530
Gross Unrealized Loss	31	36
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	8	8
Fair Value	40,455	23,760
Gross Unrealized Loss	31	22
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign | 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 | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		1
Fair Value		4,652
Gross Unrealized Loss		3
Fixed maturities | States, municipalities and political subdivisions
Gain (Loss) on Investments [Line Items]
Number of Securities	62	14
Fair Value	132,758	24,207
Gross Unrealized Loss	1,744	380
Fixed maturities | States, municipalities and political subdivisions | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	58	10
Fair Value	129,906	21,299
Gross Unrealized Loss	1,682	325
Fixed maturities | States, municipalities and political subdivisions | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	4
Fair Value	2,852	2,908
Gross Unrealized Loss	62	55
Fixed maturities | Agency mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	25	12
Fair Value	54,575	64,187
Gross Unrealized Loss	640	204
Fixed maturities | Agency mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	22	10
Fair Value	46,654	62,516
Gross Unrealized Loss	512	174
Fixed maturities | Agency mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	3	2
Fair Value	7,921	1,671
Gross Unrealized Loss	128	30
Fixed maturities | Residential Mortgage Backed Securities
Gain (Loss) on Investments [Line Items]
Number of Securities	29	41
Fair Value	6,326	9,077
Gross Unrealized Loss	187	549
Fixed maturities | Residential Mortgage Backed Securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	5	6
Fair Value	1,673	1,825
Gross Unrealized Loss	2	22
Fixed maturities | Residential Mortgage Backed Securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	24	35
Fair Value	4,653	7,252
Gross Unrealized Loss	185	527
Fixed maturities | Asset-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	2	2
Fair Value	2,374	2,369
Gross Unrealized Loss	29	49
Fixed maturities | Asset-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1
Fair Value	631
Gross Unrealized Loss	2
Fixed maturities | Asset-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	2
Fair Value	1,743	2,369
Gross Unrealized Loss	27	49
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Number of Securities	7	12
Fair Value	2,668	3,484
Gross Unrealized Loss	29	18
Fixed maturities | Commercial mortgage-backed securities | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	7
Fair Value	1,149	2,639
Gross Unrealized Loss	5	7
Fixed maturities | Commercial mortgage-backed securities | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	2
Fair Value	802
Gross Unrealized Loss	11
Fixed maturities | Commercial mortgage-backed securities | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	5
Fair Value	717	845
Gross Unrealized Loss	13	11
Fixed maturities | Corporate bonds
Gain (Loss) on Investments [Line Items]
Number of Securities	11	6
Fair Value	17,145	10,217
Gross Unrealized Loss	124	25
Fixed maturities | Corporate bonds | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	10	2
Fair Value	15,648	3,528
Gross Unrealized Loss	121	6
Fixed maturities | Corporate bonds | > 12 months
Gain (Loss) on Investments [Line Items]
Number of Securities	1	4
Fair Value	1,497	6,689
Gross Unrealized Loss	3	19
Common Stock
Gain (Loss) on Investments [Line Items]
Number of Securities	4	14
Fair Value	8,011	25,288
Gross Unrealized Loss	247	626
Common Stock | 0-6 months
Gain (Loss) on Investments [Line Items]
Number of Securities	4	13
Fair Value	8,011	23,345
Gross Unrealized Loss	247	522
Common Stock | 7-12 months
Gain (Loss) on Investments [Line Items]
Number of Securities		1
Fair Value		1,943
Gross Unrealized Loss		$ 104