Reserves (Roll Forward of Claims and Benefits Payable) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	May 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Claims and benefits payable, at beginning of period	$ 3,782.2	$ 3,301.2
Less: Reinsurance ceded and other	(3,193.3)	(2,718.2)
Net claims and benefits payable, at beginning of period	588.9	583.0
Acquired reserves as of Acquisition Date				$ 142.9	$ 0.0
Incurred losses and loss adjustment expenses related to:
Current year	938.1	856.3
Prior years	(30.6)	(60.0)
Total incurred losses and loss adjustment expenses	907.5	796.3
Paid losses and loss adjustment expenses related to:
Current year	624.6	556.0
Prior years	336.4	262.8
Total paid losses and loss adjustment expenses	961.0	818.8
Net claims and benefits payable, at end of period	678.3	560.5
Reinsurance ceded and other	3,193.3	2,718.2		3,193.3	2,718.2
Claims and benefits payable, at end of period	3,503.1	3,093.3
Claims and benefits payable	3,503.1	3,301.2		$ 3,782.2	$ 3,301.2
Plus: Reinsurance ceded and other	$ 2,824.8	$ 2,532.8
TWG Holdings Limited
Paid losses and loss adjustment expenses related to:
Reinsurance ceded and other			$ 280.6
Claims and benefits payable			$ 423.5