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Investments - Other Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	$ 3.9	$ 8.7	$ 12.1	$ 8.9
Initial investment		3.3	2.3	3.3
Goodwill	0.0	0.0	(1.8)	(22.1)
Additional investment	0.1		0.1
Distribution received			(9.3)	0.0
Unrealized/realized gain/(loss)		1.0	0.6	0.8
Closing value of investment	4.0	13.0	4.0	13.0
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.4	0.6	0.5	0.8
Initial investment		0.0	0.0	0.0
Goodwill			0.0
Additional investment	0.1		0.1
Distribution received			0.0
Unrealized/realized gain/(loss)		0.0	(0.1)	(0.2)
Closing value of investment	0.5	0.6	0.5	0.6
Chaspark
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.0	8.1	8.4	8.1
Initial investment		0.0	0.0	0.0
Goodwill			0.0
Additional investment	0.0		0.0
Distribution received			(9.3)
Unrealized/realized gain/(loss)		1.0	0.9	1.0
Closing value of investment	0.0	9.1	0.0	9.1
Bene
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	3.0	0.0	3.2
Initial investment		3.3	0.0
Goodwill			0.0
Additional investment	0.0		0.0
Distribution received			0.0
Unrealized/realized gain/(loss)		0.0	(0.2)
Closing value of investment	3.0	$ 3.3	3.0	$ 3.3
Digital Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.5		0.0
Initial investment			2.3
Goodwill			(1.8)
Additional investment	0.0		0.0
Distribution received			0.0
Unrealized/realized gain/(loss)			0.0
Closing value of investment	$ 0.5		$ 0.5

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Equity Method Investment, Aggregate Cost [Roll Forward]

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