Fair Value Measurements - Summary of Carrying or Reported Values and Corresponding Fair Values for Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Of Financial Instruments [Line Items]
Fixed maturity securities-held-to-maturity	$ 594,531	$ 627,349
Fixed maturity securities-available-for-sale	212,872	155,905
Carrying Value [Member]
Fair Value Of Financial Instruments [Line Items]
Fixed maturity securities-held-to-maturity	565,874	583,287
Fixed maturity securities-available-for-sale	199,374	149,139
Equity securities	13,498	6,766
Cash and cash equivalents	83,424	92,676
Short-term Investments	81,935	68,924
Fair Value [Member]
Fair Value Of Financial Instruments [Line Items]
Fixed maturity securities-held-to-maturity	594,531	627,349
Fixed maturity securities-available-for-sale	199,374	149,139
Equity securities	13,498	6,766
Cash and cash equivalents	83,424	92,676
Short-term Investments	$ 81,935	$ 68,924