Insurance Liabilities - Liability for Unpaid Claims (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in unpaid claims
Balance at beginning of period, unpaid claims	$ 2,001.3	$ 1,872.2	$ 1,771.4
Balance at beginning of period, reinsurance recoverables for unpaid claims	340.3	314.1	284.6
Net balance at beginning of period, unpaid claims	1,661.0	1,558.1	1,486.8
Incurred:
Incurred: Current year	1,196.6	1,103.5	1,037.0
Incurred: Prior years	18.2	24.4	(18.1)
Total incurred	1,214.8	1,127.9	1,018.9
Payments:
Payments: Current year	767.2	701.9	646.7
Payments: Prior years	353.9	323.1	300.9
Total payments	1,121.1	1,025.0	947.6
Net balance at end of period, unpaid claims	1,754.7	1,661.0	1,558.1
Balance at end of period, reinsurance recoverables for unpaid claims	375.8	340.3	314.1
Balance at end of period, unpaid claims	2,130.5	2,001.3	1,872.2
Amount not included in the rollforward above:
Claim adjustment expense liabilities	$ 50.7	$ 49.3	$ 58.9