Reserves (Roll Forward of Claims and Benefits Payable) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Reinsurance recoverables	$ 9,790.5
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Claims and benefits payable, at beginning of year	3,301.2	$ 3,896.7	$ 3,698.6
Less: Reinsurance ceded and other	(3,193.3)	(2,718.2)	(1,496.5)	$ (1,254.4)
Net claims and benefits payable, at beginning of year	583.0	2,400.2	2,444.2
Incurred losses and loss adjustment expenses related to:
Incurred losses and loss adjustment expenses, current year	1,965.0	2,028.9	4,973.4
Incurred losses and loss adjustment expenses, prior year's interest	0.0	9.8	59.8
Incurred losses and loss adjustment expenses, prior year(s)	(58.5)	(196.2)	(150.9)
Total incurred losses and loss adjustment expenses	1,906.5	1,842.5	4,882.3
Paid losses and loss adjustment expenses related to:
Paid losses and loss adjustment expenses, current year	1,536.4	1,595.7	3,846.0
Paid losses and loss adjustment expenses, prior year(s)	364.2	2,064.0	1,080.3
Total paid losses and loss adjustment expenses	1,900.6	3,659.7	4,926.3
Net claims and benefits payable, at end of year	588.9	583.0	2,400.2
Plus: Reinsurance ceded and other	3,193.3	2,718.2	1,496.5	$ 1,254.4
Claims and benefits payable, at end of year	3,782.2	3,301.2	3,896.7
Not Rated
Effects of Reinsurance [Line Items]
Reinsurance recoverables	2,622.5
Ceded To U.S. Government | Not Rated
Effects of Reinsurance [Line Items]
Reinsurance recoverables	555.0	153.3	64.0
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Claims and benefits payable, at beginning of year	153.3	64.0
Paid losses and loss adjustment expenses related to:
Claims and benefits payable, at end of year	$ 555.0	$ 153.3	$ 64.0