Insurance Liabilities - Liability for Unpaid Claims (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in unpaid claims
Balance at beginning of period, unpaid claims	$ 2,365.5	$ 2,252.7	$ 2,130.5
Balance at beginning of period, reinsurance recoverables for unpaid claims	403.8	404.3	375.8
Net balance at beginning of period, unpaid claims	1,961.7	1,848.4	1,754.7
Incurred:
Incurred: Current year	1,376.8	1,361.3	1,268.8
Incurred: Prior years	26.6	0.8	0.3
Total incurred	1,403.4	1,362.1	1,269.1
Payments:
Payments: Current year	863.8	869.4	815.7
Payments: Prior years	403.3	379.4	359.7
Total payments	1,267.1	1,248.8	1,175.4
Net balance at end of period, unpaid claims	2,098.0	1,961.7	1,848.4
Balance at end of period, reinsurance recoverables for unpaid claims	436.9	403.8	404.3
Balance at end of period, unpaid claims	2,534.9	2,365.5	2,252.7
Amount not included in the rollforward above:
Claim adjustment expense liabilities	$ 57.8	$ 57.9	$ 54.6