Segment Information (Net Premiums Earned By Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Total net premiums earned	$ 602.9	$ 646.4	$ 1,183.0	$ 1,214.9
Operating Segments [Member] | North American Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	329.6	328.6	645.9	641.5
Operating Segments [Member] | North American Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	103.1	94.7	204.6	183.5
Operating Segments [Member] | North American Insurance [Member] | General Casualty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	104.3	109.2	201.1	214.1
Operating Segments [Member] | North American Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	43.3	36.7	82.9	70.9
Operating Segments [Member] | North American Insurance [Member] | General Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	34.6	42.5	68.3	83.9
Operating Segments [Member] | North American Insurance [Member] | Other specialty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	28.1	17.3	53.3	32.9
Operating Segments [Member] | North American Insurance [Member] | Healthcare [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	16.2	28.2	35.7	56.2
Operating Segments [Member] | Global Markets Insurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	94.1	102.6	188.3	152.7
Operating Segments [Member] | Global Markets Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	25.5	26.1	49.0	49.3
Operating Segments [Member] | Global Markets Insurance [Member] | General Casualty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	22.2	31.0	45.8	37.0
Operating Segments [Member] | Global Markets Insurance [Member] | General Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	16.2	18.3	32.5	26.5
Operating Segments [Member] | Global Markets Insurance [Member] | Other specialty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	30.2	27.2	61.0	39.9
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	179.2	215.2	348.8	420.7
Operating Segments [Member] | Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	83.4	104.8	167.8	209.1
Operating Segments [Member] | Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	46.7	58.9	92.8	115.8
Operating Segments [Member] | Reinsurance [Member] | Specialty [Member]
Segment Reporting Information [Line Items]
Total net premiums earned	$ 49.1	$ 51.5	$ 88.2	$ 95.8