Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring basis - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 1,004.7	$ 800.8
Short-term investment securities	102.8	100.9
Total assets	1,107.5	901.7
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	1,004.7	800.8
Short-term investment securities	102.8	100.9
Total assets	1,107.5	901.7
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
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
Total assets	0.0	0.0
Total liabilities	$ 0.0	$ 0.0