Cash Equivalents and Investments (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Line Items]
Fair Value	$ 7.7	$ 33.3
Cash and cash equivalents	1,178.0	1,685.0
Short-term investments	95.8	132.5
Money market mutual funds
Cash and Cash Equivalents [Line Items]
Cost		15.2
Gross Unrealized Gains		0.0
Fair Value		15.2
Cash and cash equivalents		15.2
Short-term investments		0.0
Other assets		0.0
Certificates of deposit and money market deposit accounts
Cash and Cash Equivalents [Line Items]
Cost	436.3	1,022.4
Gross Unrealized Gains	0.0	0.0
Fair Value	436.3	1,022.4
Cash and cash equivalents	327.6	904.3
Short-term investments	88.2	115.8
Other assets	20.5	2.3
Open ended mutual funds
Cash and Cash Equivalents [Line Items]
Cost	6.8	29.5
Gross Unrealized Gains	0.9	3.8
Fair Value	7.7	33.3
Cash and cash equivalents	0.0	0.0
Short-term investments	7.6	16.7
Other assets	$ 0.1	$ 16.6