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Variable Interest Entities Summary of Silverton Loan Notes (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Silverton Loan Notes [Roll Forward]
Opening balance				$ 277.9				$ 235.0					$ 277.9	$ 235.0
Total change in fair value for the period	$ 17.6	$ 9.8	$ (3.3)	(2.9)	$ (3.4)	$ (9.8)	$ 0.5	(4.4)	$ (5.3)	$ (8.3)	$ (3.3)	$ (2.9)	21.2	(17.1)	$ (19.8)
Total distributed in the period													(144.2)	(108.5)
Total issued in the period														130.0
Closing balance as at December 31, 2017	107.2				277.9				235.0				107.2	277.9	235.0
Loan notes (long-term liabilities)	54.7				142.0								54.7	142.0
Accrued expenses (current liabilities)	52.5				135.9								52.5	135.9
Third party
Movement in Silverton Loan Notes [Roll Forward]
Opening balance				223.4				190.6					223.4	190.6
Total change in fair value for the period													(21.2)	17.1
Total distributed in the period													(115.6)	(89.3)
Total issued in the period														105.0
Closing balance as at December 31, 2017	86.6				223.4				190.6				86.6	223.4	190.6
Loan notes (long-term liabilities)	44.2				115.0								44.2	115.0
Accrued expenses (current liabilities)	42.4				108.4								42.4	108.4
Aspen Holdings
Movement in Silverton Loan Notes [Roll Forward]
Opening balance				$ 54.5				$ 44.4					54.5	44.4
Total change in fair value for the period													(5.3)	4.3
Total distributed in the period													(28.6)	(19.2)
Total issued in the period													0.0	25.0
Closing balance as at December 31, 2017	20.6				54.5				$ 44.4				20.6	54.5	$ 44.4
Loan notes (long-term liabilities)	10.5				27.0								10.5	27.0
Accrued expenses (current liabilities)	$ 10.1				$ 27.5								10.1	27.5
Aspen Holdings | Third party
Movement in Silverton Loan Notes [Roll Forward]
Total change in fair value for the period													$ (26.5)	$ 21.4

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