Statutory surplus and dividend restrictions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2019
Equity [Abstract]
Dividends paid by guaranty	$ 25.0	$ 20.0	$ 20.0
Surplus for guaranty	574.8	542.7
Statutory net income	74.2	$ 47.7	$ 53.6
Dividends Payable [Line Items]
Maximum amount of dividend to be paid	108.5
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	$ 2.0
Scenario, Forecast
Dividends Payable [Line Items]
Maximum amount of dividend to be paid				$ 115.0