Fair Value - Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets and Liabilities Measured at fair value on a recurring basis
Restricted cash	$ 0.8	$ 0.8
Fair Value, Recurring
Assets and Liabilities Measured at fair value on a recurring basis
Cash and cash equivalents	261.8	320.3
Restricted cash	0.8	0.8
Short-term investments	43.6	37.6
Foreign currency forward contracts	2.7	1.1
Total Assets	308.9	359.8
Foreign currency forward contracts	0.3	8.1
Total Liabilities	0.3	8.1
Fair Value, Recurring | Level 1
Assets and Liabilities Measured at fair value on a recurring basis
Cash and cash equivalents	260.8	318.6
Restricted cash	0.8	0.8
Short-term investments	0.0	0.0
Foreign currency forward contracts	0.0	0.0
Total Assets	261.6	319.4
Foreign currency forward contracts	0.0	0.0
Total Liabilities	0.0	0.0
Fair Value, Recurring | Level 2
Assets and Liabilities Measured at fair value on a recurring basis
Cash and cash equivalents	1.0	1.7
Restricted cash	0.0	0.0
Short-term investments	43.6	37.6
Foreign currency forward contracts	2.7	1.1
Total Assets	47.3	40.4
Foreign currency forward contracts	0.3	8.1
Total Liabilities	0.3	8.1
Fair Value, Recurring | Level 3
Assets and Liabilities Measured at fair value on a recurring basis
Cash and cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Short-term investments	0.0	0.0
Foreign currency forward contracts	0.0	0.0
Total Assets	0.0	0.0
Foreign currency forward contracts	0.0	0.0
Total Liabilities	$ 0.0	$ 0.0