Reserves - Roll Forward of Claims and Benefits Payable (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Claims and benefits payable	$ 2,687.7	$ 2,722.2	$ 2,669.1	$ 2,722.2
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Claims and benefits payable, at beginning of period	2,687.7	2,813.7
Less: Reinsurance ceded and other	(1,900.0)	(2,053.7)
Net claims and benefits payable, at beginning of period	787.7	760.0
Incurred losses and loss adjustment expenses related to:
Current year	1,242.1	1,339.4
Prior years	(43.1)	(37.7)
Total incurred losses and loss adjustment expenses	1,199.0	1,301.7
Paid losses and loss adjustment expenses related to:
Current year	786.9	882.9
Prior years	399.4	409.2
Total paid losses and loss adjustment expenses	1,186.3	1,292.1
Net claims and benefits payable, at end of period	800.4	769.6
Plus: Reinsurance ceded and other	1,868.7	1,952.6
Claims and benefits payable, at end of period	$ 2,669.1	$ 2,722.2
Ceded to U.S. Government | Not Rated
Paid losses and loss adjustment expenses related to:
Reinsurance recoverables			$ 72.0	$ 83.2