Investments - Investments in Privately-Held Companies (Details) - Privately-held Companies $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)
Investments in Privately-Held Companies
Investments in Privately-Held Companies	$ 142.5
Upward price changes of investments	0.0
Downward price changes of investments	$ 0.0

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