Marketable Securities - Additional Information (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Investments, Debt and Equity Securities [Abstract]
Realized gain (loss) on marketable securities	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Debt securities in an unrealized loss position \| security	68		68		242
Aggregate fair value of debt securities in an unrealized loss position	$ 159.8		$ 159.8		$ 639.3

Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

No definition available.

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant