Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 3.0	$ 6.0
Increase (decrease) in estimates for claims	4.0	(3.0)
Adjustment expense	4.0	6.7
Liability recognized for best estimate net of reinsurance	67.0
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Favorable reserve development net of premium offsets	7.0	9.6
Liability recognized for best estimate net of reinsurance	47.0
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	3.0	$ 2.9
Liability recognized for best estimate net of reinsurance	$ 20.0