Unpaid Loss and Loss Adjustment Expense - Claims Development Information, Paid Losses (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Claims Development [Line Items]
Total paid losses and ALAE, net	$ 104.6
Total unpaid loss and ALAE reserves, net	10.0
Ceded unpaid loss and LAE	36.3	$ 18.5	$ 11.3
Unpaid losses and loss adjustment expenses	46.3	28.2	13.1
Accident Year 2016
Claims Development [Line Items]
Total paid losses and ALAE, net	0.0	0.0	0.0	$ 0.0	$ 0.0
Accident Year 2017
Claims Development [Line Items]
Total paid losses and ALAE, net	1.7	1.7	1.7	$ 1.6
Accident Year 2018
Claims Development [Line Items]
Total paid losses and ALAE, net	13.4	13.4	$ 13.2
Accident Year 2019
Claims Development [Line Items]
Total paid losses and ALAE, net	46.1	$ 36.4
Accident Year 2020
Claims Development [Line Items]
Total paid losses and ALAE, net	$ 43.4