Investments (Schedule Of Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Net Investment Income [Line Items]
Net investment income	$ 55.8	$ 42.8	$ 109.1	$ 87.3
Fixed maturity investments [Member]
Net Investment Income [Line Items]
Net investment income	48.9	41.2	96.9	77.4
Equity securities [Member]
Net Investment Income [Line Items]
Net investment income	1.4	4.9	3.3	8.4
Other invested assets: hedge funds and private equity [Member]
Net Investment Income [Line Items]
Net investment income	7.6	4.6	12.3	13.0
Other invested assets: other private securities [Member]
Net Investment Income [Line Items]
Net investment income	1.7	(3.8)	4.7	(2.9)
Cash and cash equivalents [Member]
Net Investment Income [Line Items]
Net investment income	0.8	0.4	1.3	0.9
Expenses [Member]
Net Investment Income [Line Items]
Net investment income	$ (4.6)	$ (4.5)	$ (9.4)	$ (9.5)