Fair Value Measurements - Summary of Carrying Values and Corresponding Fair Values for Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value Of Financial Instruments [Line Items]
Fixed maturity securities-held-to-maturity	$ 576,989	$ 627,349
Fixed maturity securities-held-to-maturity	550,153	583,287
Fixed maturity securities-available-for-sale	202,701	149,139
Equity securities	12,260	6,766
Carrying Amount [Member]
Fair Value Of Financial Instruments [Line Items]
Fixed maturity securities-held-to-maturity	550,153	583,287
Fixed maturity securities-available-for-sale	202,701	149,139
Equity securities	12,260	6,766
Cash and cash equivalents	116,778	92,676
Short-term Investments	86,131	68,924
Other Investments	10,271
Fair Value [Member]
Fair Value Of Financial Instruments [Line Items]
Fixed maturity securities-held-to-maturity	576,989	627,349
Fixed maturity securities-available-for-sale	202,701	149,139
Equity securities	12,260	6,766
Cash and cash equivalents	116,778	92,676
Short-term Investments	86,131	68,924
Other Investments	$ 10,271