Income Taxes (Sources of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes	$ (54.1)	$ 65.4	$ 159.7	$ 75.2	$ 5.8	$ (57.0)	$ 9.6	$ 131.3	$ 246.2	$ 89.7	$ 520.8
Switzerland
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes									(25.5)	(15.7)	(19.0)
Bermuda
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes									336.6	162.1	460.2
United States
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes									(27.5)	12.8	72.9
All other jurisdictions
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes									$ (37.4)	$ (69.5)	$ 6.7