Statutory surplus and dividend restrictions (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Dividends Payable [Line Items]
Maximum amount of dividend to be paid	$ 115,000,000
Dividends paid by guaranty	0	$ 25,000,000.0	$ 20,000,000.0
Surplus for guaranty	617,900,000	574,800,000
Statutory net income	38,300,000	$ 74,200,000	$ 47,700,000
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	2,000,000.0
Underwriter Entities
Dividends Payable [Line Items]
Statutory capital and surplus necessary to satisfy regulatory requirements for guaranty	$ 2,000,000.0
Scenario, Forecast
Dividends Payable [Line Items]
Maximum amount of dividend to be paid				$ 115,000,000.0