Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
(Favorable) unfavorable loss and LAE development	$ (7.4)	$ (18.7)
Net reserves	7,103.0	6,722.3	$ 6,949.4	$ 6,574.4
Commercial Lines [Member]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
(Favorable) unfavorable loss and LAE development		19.8
Other Commercial Lines [Member]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
(Favorable) unfavorable loss and LAE development		15.5
Chaucer
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
(Favorable) unfavorable loss and LAE development	$ (7.7)	(39.4)
Year 2014 to 2015 [Member] | Chaucer | Treaty Line [Member]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
(Favorable) unfavorable loss and LAE development		$ (11.1)