Statutory surplus and dividend restrictions (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Maximum amount of dividend to be paid	$ 102.0
Dividends paid by guaranty	20.0	$ 15.0	$ 25.0
Surplus for guaranty	509.9	501.8
Statutory net income	53.6	$ 83.2	$ 53.2
Dividends Payable [Line Items]
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	2.0
Underwriter entities
Dividends Payable [Line Items]
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	$ 15.0