Investments - Cost and Equity Method Investments (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Jun. 27, 2015
Schedule of Equity Method Investments [Line Items]
Cost-method investments, other than temporary impairment		$ 1.0
Net gain (loss) from equity method investment	$ 5.4	0.3	$ (4.1)	$ 11.6
Transfer from investments			15.5
Unrealized gain (loss) on investments			8.7
Equity method investment other than temporary impairment			22.3
Other noncurrent assets
Schedule of Equity Method Investments [Line Items]
Cost method Investments	6.9	6.3	6.9
Equity method investments	$ 45.5	$ 4.9	$ 4.6