Insurance Liabilities - Liability for Unpaid Claims (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Change in unpaid claims
Balance at beginning of period, unpaid claims	$ 2,534.9	$ 2,365.5
Balance at beginning of period, reinsurance recoverables for unpaid claims	436.9	403.8
Net balance at beginning of period, unpaid claims	2,098.0	1,961.7
Incurred:
Incurred: Current year	736.0	985.5
Incurred: Prior years	441.7	37.9
Total incurred	1,177.7	1,023.4
Payments:
Payments: Current year	313.6	595.7
Payments: Prior years	773.2	335.1
Total payments	1,086.8	930.8
Net balance at end of period, unpaid claims	2,188.9	2,054.3
Balance at end of period, reinsurance recoverables for unpaid claims	437.9	425.4
Balance at end of period, unpaid claims	2,626.8	2,479.7
Amount not included in the rollforward above:
Claim adjustment expense liabilities	$ 59.5	$ 58.0