Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring basis - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 800.8	$ 700.9
Short-term investment securities	100.9	100.2
Total assets	901.7	801.1
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	800.8	700.9
Short-term investment securities	100.9	100.2
Total assets	901.7	801.1
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