Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 1,947.6	$ 979.0
Restricted cash	54.8
Short-term investment securities	106.3	105.3
Assets held for sale	2.3	2.3
Total assets	2,111.0	1,086.6
Total liabilities	0.0	0.0
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,947.6	979.0
Restricted cash	54.8
Short-term investment securities	106.3	105.3
Assets held for sale	0.0	0.0
Total assets	2,108.7	1,084.3
Total liabilities	0.0	0.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Restricted cash	0.0
Short-term investment securities	0.0	0.0
Assets held for sale	0.0	0.0
Total assets	0.0	0.0
Total liabilities	0.0	0.0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Restricted cash	0.0
Short-term investment securities	0.0	0.0
Assets held for sale	2.3	2.3
Total assets	2.3	2.3
Total liabilities	$ 0.0	$ 0.0