Statutory surplus and dividend restrictions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2021
Dividends Payable [Line Items]
Dividends paid by guaranty	$ 30.0		$ 25.0
Surplus for guaranty	794.7	$ 617.9
Statutory net income	133.2	$ 38.3	$ 74.2
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
Forecast
Dividends Payable [Line Items]
Maximum amount of dividend to be paid				$ 158.9