Cash Equivalent and Investments (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Fair value	$ 15.1	$ 33.3
Cash and cash equivalents	1,195.9	1,685.0	$ 1,314.5	$ 1,071.5
Short-term investments	119.5	132.5
Money Market [Member]
Cash and Cash Equivalents [Line Items]
Cost		15.2
Fair value		15.2
Cash and cash equivalents		15.2
Certificates Of Deposit [Member]
Cash and Cash Equivalents [Line Items]
Cost	437.5	1,022.4
Fair value	437.5	1,022.4
Cash and cash equivalents	327.4	904.3
Short-term investments	104.5	115.8
Other assets	5.6	2.3
Open Ended Mutual Funds [Member]
Cash and Cash Equivalents [Line Items]
Cost	13.5	29.5
Gross unrealized gains	1.6	3.8
Fair value	15.1	33.3
Short-term investments	15.0	16.7
Other assets	$ 0.1	$ 16.6