Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 1,122.1	$ 1,004.7
Short-term investment securities	103.5	102.8
Total assets	1,225.6	1,107.5
Total liabilities	0.0	0.0
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,122.1	1,004.7
Short-term investment securities	103.5	102.8
Total assets	1,225.6	1,107.5
Total liabilities	0.0	0.0
Level 2 [Member]
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 [Member]
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