Liability for Unpaid Losses and Loss Adjustment Expenses Historical Incurred Losses and LAE (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)		Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)	[1]	Dec. 31, 2017 USD ($)	[1]	Dec. 31, 2016 USD ($)	[1]	Dec. 31, 2015 USD ($)	[1]	Dec. 31, 2014 USD ($)	[1]	Dec. 31, 2013 USD ($)	[1]	Dec. 31, 2012 USD ($)	[1]
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	$ 3,848.0
Cumulative paid losses and LAE, net of reinsurance	2,781.1
Liabilities for unpaid losses and LAE, net of reinsurance	1,441.3
Prior years	(39.8)	$ (81.6)		$ (77.5)
Involuntary assigned risk business
Claims Development [Line Items]
Prior years	(1.8)	(1.4)
Accident years prior to 2011
Claims Development [Line Items]
Cumulative paid losses and LAE, net of reinsurance	374.4
Accident year 2012
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	369.4	372.4	[1]	378.5	[1]	$ 379.8		$ 382.8		$ 388.2		$ 386.4		$ 360.9		$ 359.9		$ 348.8
IBNR	$ 23.5
Cumulative number of reported claims	26,046
Cumulative paid losses and LAE, net of reinsurance	$ 332.3	328.4	[1]	324.3	[1]	316.9		305.0		289.9		261.4		214.2		148.3		$ 58.6
Accident year 2013
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	440.9	446.8	[1]	459.3	[1]	464.6		468.9		472.6		478.6		460.6		452.6
IBNR	$ 28.2
Cumulative number of reported claims	28,927
Cumulative paid losses and LAE, net of reinsurance	$ 391.3	386.6	[1]	379.3	[1]	365.9		346.1		317.4		263.8		184.4		$ 68.5
Accident Year 2014
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	406.0	415.5	[1]	424.7	[1]	430.5		434.6		432.9		445.8		463.4
IBNR	$ 34.6
Cumulative number of reported claims	28,594
Cumulative paid losses and LAE, net of reinsurance	$ 357.7	351.4	[1]	342.1	[1]	323.4		297.2		248.9		172.7		$ 65.3
Accident Year 2015
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	384.9	396.7	[1]	408.7	[1]	419.6		423.9		425.8		422.2
IBNR	$ 33.4
Cumulative number of reported claims	27,264
Cumulative paid losses and LAE, net of reinsurance	$ 329.3	322.2	[1]	311.2	[1]	290.5		246.9		174.5		$ 65.5
Accident year 2016
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	354.8	364.6	[1]	375.0	[1]	395.4		414.6		419.0
IBNR	$ 35.6
Cumulative number of reported claims	25,806
Cumulative paid losses and LAE, net of reinsurance	$ 290.0	278.3	[1]	261.2	[1]	227.7		166.8		$ 65.6
Accident year 2017
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	329.8	337.9	[1]	358.3	[1]	391.3		412.4
IBNR	$ 43.0
Cumulative number of reported claims	25,091
Cumulative paid losses and LAE, net of reinsurance	$ 260.0	243.7	[1]	215.7	[1]	160.2		$ 63.5
Accident Year 2018
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	400.6	407.7	[1]	424.6	[1]	422.5
IBNR	$ 52.2
Cumulative number of reported claims	27,977
Cumulative paid losses and LAE, net of reinsurance	$ 293.6	254.2	[1]	189.9	[1]	$ 77.9
Accident Year 2019
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	448.5	435.7	[1]	422.4	[1]
IBNR	$ 83.2
Cumulative number of reported claims	32,885
Cumulative paid losses and LAE, net of reinsurance	$ 285.2	212.6	[1]	$ 88.8	[1]
Prior years	10.0	13.3
Accident Year 2020
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	374.0	365.7	[1]
IBNR	$ 94.6
Cumulative number of reported claims	23,948
Cumulative paid losses and LAE, net of reinsurance	$ 175.6	$ 71.9	[1]
Prior years	8.0
Short-Duration Insurance Contract, Accident Year 2021
Claims Development [Line Items]
Incurred losses and LAE, net of reinsurance	339.2
IBNR	$ 163.0
Cumulative number of reported claims	19,287
Cumulative paid losses and LAE, net of reinsurance	$ 66.1

[1] Data presented for these calendar years is required supplementary information, which is unaudite