Reserve For Losses And Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Effect of Change in Valuation Method, Loss Reserves	$ (20.1)	$ (92.6)	$ (159.7)