Fair Value Measurements - Schedule of Assets and Liabilities Measured at Amortized Cost (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 607,512	$ 561,179
States and political subdivisions [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	414,136	399,155
Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	103,624	68,243
Commercial mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	52,094	54,244
U.S. agency-based mortgage-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	21,062	23,565
US Treasury securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	7,339	6,602
Obligations of U.S. government agencies [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,051	5,914
Asset-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	3,206	3,456
Fair Value, Inputs, Level 1 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	7,339	6,602
Fair Value, Inputs, Level 1 [Member] | US Treasury securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	7,339	6,602
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	600,173	554,577
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	414,136	399,155
Fair Value, Inputs, Level 2 [Member] | Corporate bonds [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	103,624	68,243
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	52,094	54,244
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	21,062	23,565
Fair Value, Inputs, Level 2 [Member] | Obligations of U.S. government agencies [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	6,051	5,914
Fair Value, Inputs, Level 2 [Member] | Asset-backed securities [Member]
Fair Value Measurements Disclosure [Line Items]
Securities held-to-maturity-fixed maturity, Fair Value	$ 3,206	$ 3,456