Investments - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Apr. 30, 2017
Investments in Privately-Held Companies and Investments in Equity Securities with Readily Determinable Fair Values
Impairment charges			$ 12.4		$ 12.4
Goodwill on deconsolidation	$ 3.5
Privately-held Companies
Investments in Privately-Held Companies and Investments in Equity Securities with Readily Determinable Fair Values
Investments	101.7			$ 101.7
Upward price changes of investments	0.0			0.0
Impairment charges					$ 12.4
Privately-held Companies | Variable Interest Entity
Investments in Privately-Held Companies and Investments in Equity Securities with Readily Determinable Fair Values
Minority equity investment in a privately-held company							$ 7.5
Impairment charge on research and development		$ 8.8
Goodwill on deconsolidation	3.5
Temporary equity derecognized		$ 8.4
Net loss on deconsolidating	(2.0)
Equity securities
Investments in Privately-Held Companies and Investments in Equity Securities with Readily Determinable Fair Values
Investments in equity securities with readily determinable fair value	$ 64.8			$ 64.8		$ 3.5