Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 11.0	$ 13.0
Increase (decrease) in estimates for claims	9.0	(6.0)
Adjustment expense	14.0	19.0
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	13.6	19.3
Favorable reserve development net of premium offsets	17.2	18.6
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	$ 3.6	$ 0.7