Statutory Surplus and Dividend restrictions - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Dividends Payable [Line Items]
Maximum amount of dividend to be paid	$ 105,200,000
Dividends paid by Guaranty	25,000,000	0	0
Surplus for guaranty	525,800,000	473,700,000
Statutory net income (losses)	53,200,000	43,000,000	10,500,000
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	2,000,000
Guaranty recorded surplus	1,000,000	27,200,000
Maximum | Underwriter entities
Dividends Payable [Line Items]
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	$ 15,000,000