Statutory surplus and dividend restrictions - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Dividends Payable [Line Items]
Maximum amount of dividend to be paid	$ 100,400,000
Dividends paid by Guaranty	15,000,000	$ 25,000,000	$ 0
Surplus for guaranty	501,800,000	525,800,000
Statutory net income	83,200,000	$ 53,200,000	$ 43,000,000
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	2,000,000
Underwriter entities
Dividends Payable [Line Items]
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	$ 15,000,000