Statutory surplus and dividend restrictions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
Equity [Abstract]
Dividends paid by guaranty	$ 20.0	$ 20.0	$ 15.0
Surplus for guaranty	542.7	509.9
Statutory net income	47.7	$ 53.6	$ 83.2
Dividends Payable [Line Items]
Maximum amount of dividend to be paid	102.0
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
Scenario, Forecast
Dividends Payable [Line Items]
Maximum amount of dividend to be paid				$ 108.5