Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 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	807,000	2,193,000	2,323,000	8,529,000
Portfolio duration of investments			3 years 10 months 24 days	3 years 9 months 18 days
Net realized gains(losses) from sale of investments	(988,000)	4,761,000	7,171,000	10,820,000
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	4.6		4.6
Non-government backed securities | Maximum
Schedule of Investments [Line Items]
Percentage of investment	5.00%		5.00%		5.00%
Non-government backed securities | Investments
Schedule of Investments [Line Items]
Gross Unrealized Loss			$ 1,000,000		$ 200,000
Equity Securities | Maximum
Schedule of Investments [Line Items]
Percentage of fair value over cost	80.00%		80.00%