Investments - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Schedule of Investments [Line Items]
Non-credit OTTI on debt securities in an unrealized gain position	$ 400,000		$ 20,000
Investment Expenses	729,000	5,412,000
Portfolio duration of investments	4 years	3 years 8 months 12 days
Net realized gains from sale of investments	4,814,000	1,842,000
Settlement Agreement
Schedule of Investments [Line Items]
Investment Expenses	9,200,000	4,500,000
Mortgage-backed and asset-backed securities
Schedule of Investments [Line Items]
Securities, effective maturities	4.3
Non-government backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%
Non-government backed securities | Investments
Schedule of Investments [Line Items]
Gross Unrealized Loss	$ 300,000		$ 200,000
Equity Securities | Maximum
Schedule of Investments [Line Items]
Percentage of fair value over cost	80.00%