Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring basis - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 979.0	$ 1,004.7
Short-term investment securities	105.3	102.8
Assets Held-for-sale, Long Lived, Fair Value Disclosure	2.3
Total assets	1,086.6	1,107.5
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	979.0	1,004.7
Short-term investment securities	105.3	102.8
Assets Held-for-sale, Long Lived, Fair Value Disclosure	0.0
Total assets	1,084.3	1,107.5
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
Short-term investment securities	0.0	0.0
Assets Held-for-sale, Long Lived, Fair Value Disclosure	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
Short-term investment securities	0.0	0.0
Assets Held-for-sale, Long Lived, Fair Value Disclosure	2.3
Total assets	2.3	0.0
Total liabilities	$ 0.0	$ 0.0