Fair Value Measurements -Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 576,989	$ 627,349
States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	407,582	431,968
Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	72,072	84,377
Commercial mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	55,275	57,962
U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	25,842	36,042
U.S. Treasury securities and obligations of U.S. government agencies [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	12,652	12,755
Asset-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	3,566	4,245
Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,640	6,174
Fair Value, Inputs, Level 1 [Member] | States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 1 [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 1 [Member] | Commercial mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 1 [Member] | U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury securities and obligations of U.S. government agencies [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,640	6,174
Fair Value, Inputs, Level 1 [Member] | Asset-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	570,349	621,175
Fair Value, Inputs, Level 2 [Member] | States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	407,582	431,968
Fair Value, Inputs, Level 2 [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	72,072	84,377
Fair Value, Inputs, Level 2 [Member] | Commercial mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	55,275	57,962
Fair Value, Inputs, Level 2 [Member] | U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	25,842	36,042
Fair Value, Inputs, Level 2 [Member] | U.S. Treasury securities and obligations of U.S. government agencies [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,012	6,581
Fair Value, Inputs, Level 2 [Member] | Asset-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	3,566	4,245
Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 3 [Member] | States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 3 [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 3 [Member] | Commercial mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 3 [Member] | U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 3 [Member] | U.S. Treasury securities and obligations of U.S. government agencies [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value
Fair Value, Inputs, Level 3 [Member] | Asset-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value