Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 6.0		$ 6.0
Decrease in estimates for claims	3.0	$ 7.0
Adjustment expense	7.0	12.0
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	6.7	12.2
Favorable reserve development net of premium offsets	9.6	16.3
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	$ 2.9	$ 4.1