Statutory surplus and dividend restrictions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Dividends Payable [Line Items]
Dividends paid by guaranty	$ 150.0	$ 293.9
Extraordinary dividends paid	135.0	135.0
Maximum amount of dividend to be paid	158.1
Surplus for guaranty	790.4	826.9
Statutory net income	144.9	$ 188.8	$ 133.2
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