Segment Information (Net Premiums Earned by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Net premiums earned	$ 584.0	$ 650.7	$ 1,767.1	$ 1,865.6
Operating Segments [Member] | North American Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	312.8	332.8	958.8	974.2
Operating Segments [Member] | North American Insurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	100.7	112.2	301.8	326.2
Operating Segments [Member] | North American Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	93.9	98.4	298.5	281.9
Operating Segments [Member] | North American Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Net premiums earned	42.4	38.3	125.3	109.2
Operating Segments [Member] | North American Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29.1	39.3	97.5	123.2
Operating Segments [Member] | North American Insurance [Member] | Other specialty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	30.9	20.0	84.2	52.9
Operating Segments [Member] | North American Insurance [Member] | Healthcare [Member]
Segment Reporting Information [Line Items]
Net premiums earned	15.8	24.6	51.5	80.8
Operating Segments [Member] | Global Markets Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	96.8	110.3	285.1	263.0
Operating Segments [Member] | Global Markets Insurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	22.3	32.0	68.0	68.9
Operating Segments [Member] | Global Markets Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	24.2	27.7	73.2	77.0
Operating Segments [Member] | Global Markets Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	16.8	20.6	49.3	47.2
Operating Segments [Member] | Global Markets Insurance [Member] | Other specialty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	33.5	30.0	94.6	69.9
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	174.4	207.6	523.2	628.4
Operating Segments [Member] | Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	82.9	102.3	250.6	311.5
Operating Segments [Member] | Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	48.1	58.2	141.0	174.0
Operating Segments [Member] | Reinsurance [Member] | Specialty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 43.4	$ 47.1	$ 131.6	$ 142.9