Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	0 Months Ended			1 Months Ended			3 Months Ended	12 Months Ended
	Nov. 14, 2013	Apr. 29, 2004	Dec. 16, 2003	Aug. 31, 2012	May 31, 2012	Jul. 31, 2011	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Sep. 15, 2013	Jan. 31, 2013
Schedule Of Accounting Policies [Line Items]
Percentage of comparable consolidated amounts								95.00%	95.00%	95.00%
Original maturity of cash equivalent								3 months
Decrease in underwriting and other operating costs	$ 1,500,000						$ 1,700,000
Increased income tax expense	500,000						600,000
Net income statement impact	1,000,000						1,100,000
Net income statement impact per share	$ 0.05						$ 0.06
Cash received on commutation of reinsurance agreement											14,900,000	300,000
Experience rated commission and other operating costs								9,300,000	11,100,000	5,700,000
Commutation payment adjusted through offset against balance of funds											17,600,000
Advertising expense								491,000	444,000	421,000
Premium tax benefit								3,600,000	3,400,000
Assessment Related premium Payment period								1 year
Assessment Related loss Payment period								1 year
Proceeds from issuance of trust preferred securities		25,000,000	10,000,000
Purchase of subordinated debt securities		25,800,000	10,300,000
Redemption of debt securities				12,900,000	12,900,000	10,300,000			25,780,000	10,310,000
Land and office building [Member]
Schedule Of Accounting Policies [Line Items]
Estimated useful life of assets								39 years
Limited Partnership [Member]
Schedule Of Accounting Policies [Line Items]
Unfunded commitments							$ 0	$ 0
Maximum [Member]
Schedule Of Accounting Policies [Line Items]
Short-term investments maturity period								P1Y
Minimum [Member]
Schedule Of Accounting Policies [Line Items]
Short-term investments maturity period								P3M
Other Assets [Member] | Maximum [Member]
Schedule Of Accounting Policies [Line Items]
Estimated useful life of assets								7 years
Other Assets [Member] | Minimum [Member]
Schedule Of Accounting Policies [Line Items]
Estimated useful life of assets								3 years