Unpaid Loss and Loss Adjustment Expense - Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Claims Development [Line Items]
Rollforward table, current year, incurred	$ 54.8	$ 47.3
Development table, current year, incurred	114.6
Rollforward table, current year, paid	44.6	37.7
Development table, current year, paid	104.6
Rollforward table, prior year, incurred	(0.1)	(1.5)
Rollforward table, prior year, paid	9.8	0.2
2020 Current Accident Year
Claims Development [Line Items]
Rollforward table, current year, incurred	54.8
Development table, current year, incurred	53.5
Variance, current year, incurred	1.3
Unallocated loss adjustment expense, incurred	1.3
Rollforward table, current year, paid	44.6
Development table, current year, paid	43.4
Variance, current year, paid	1.2
Unallocated loss adjustment expense, paid	1.2
2020 Prior Accident Year
Claims Development [Line Items]
Development table, current year, incurred	46.0	46.0
Development table, current year, paid	46.1	$ 36.4
Rollforward table, prior year, incurred	(0.1)
Development table, prior year, incurred	0.0
Variance, prior year, incurred	(0.1)
Rollforward table, prior year, paid	9.8
Development table, prior year, paid	9.7
Variance, prior year, paid	$ 0.1