Investments (Schedule Of Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Net Investment Income [Line Items]
Net investment income	$ 50.6	$ 45.7	$ 159.7	$ 133.0
Fixed maturity investments [Member]
Net Investment Income [Line Items]
Net investment income	46.4	42.8	143.3	120.3
Equity securities [Member]
Net Investment Income [Line Items]
Net investment income	1.1	3.1	4.4	11.5
Other invested assets - hedge funds and private equity [Member]
Net Investment Income [Line Items]
Net investment income	5.9	3.4	18.2	16.3
Other invested assets - other private securities [Member]
Net Investment Income [Line Items]
Net investment income	1.0	1.8	5.7	(1.1)
Cash and cash equivalents [Member]
Net Investment Income [Line Items]
Net investment income	0.7	0.3	2.0	1.2
Expenses [Member]
Net Investment Income [Line Items]
Net investment income	$ (4.5)	$ (5.7)	$ (13.9)	$ (15.2)