Dividend Restrictions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Dividends Payable [Line Items]
Maximum amount of dividend to be paid	$ 85.8
Surplus for guaranty	429.2	371.8
Statutory net income (losses)	$ 10.5	$ (4.2)	$ (2.0)