Insurance Liabilities - Liability for Unpaid Claims (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in unpaid claims
Balance at beginning of period, unpaid claims	$ 2,252.7	$ 2,130.5	$ 2,001.3
Balance at beginning of period, reinsurance recoverables for unpaid claims	404.3	375.8	340.3
Net balance at beginning of period, unpaid claims	1,848.4	1,754.7	1,661.0
Incurred:
Incurred: Current year	1,361.3	1,268.8	1,196.6
Incurred: Prior years	0.8	0.3	18.2
Total incurred	1,362.1	1,269.1	1,214.8
Payments:
Payments: Current year	869.4	815.7	767.2
Payments: Prior years	379.4	359.7	353.9
Total payments	1,248.8	1,175.4	1,121.1
Net balance at end of period, unpaid claims	1,961.7	1,848.4	1,754.7
Balance at end of period, reinsurance recoverables for unpaid claims	403.8	404.3	375.8
Balance at end of period, unpaid claims	2,365.5	2,252.7	2,130.5
Amount not included in the rollforward above:
Claim adjustment expense liabilities	$ 57.9	$ 54.6	$ 50.7