Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 15.0	$ 19.0
Increase (decrease) in estimates for claims	12.0	(2.0)
Adjustment expense	17.2	26.6
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable reserve development net of premium offsets	18.2	22.9
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	$ 1.0	$ 3.7