Cash, Cash Equivalents and Short-Term Investments - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) investment investment_type	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) investment investment_type	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Investment Holdings [Line Items]
Cash in transit for credit and debit card transactions	$ 13.2		$ 13.2		$ 11.5
Short-term investments, estimated fair value	$ 783.5		$ 783.5		607.7
Number of investments in unrealized loss positions | investment	52		52
Number of security types | investment_type	8		8
Number of security types in loss position | investment_type	2		2
Investment income	$ 2.7	$ 5.6	$ 7.8	$ 11.8
Corporate notes and obligations
Investment Holdings [Line Items]
Short-term investments, estimated fair value	361.4		361.4		286.6
Unrealized losses, short term investments	0.2		0.2
Asset-backed securities
Investment Holdings [Line Items]
Short-term investments, estimated fair value	83.6		83.6		$ 53.8
Unrealized losses, short term investments	$ 0.1		$ 0.1