Fair Value Measurements (Details) - Recurring fair value measurements - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Equity securities | Other income (expense), net
Liabilities
Net unrealized losses on securities	$ 13.0
Net unrealized gains on securities		$ 19.5
Level 1
Assets
Total assets	318.2	318.2	$ 251.1
Level 1 | Money market funds
Assets
Total assets	253.4	253.4	247.6
Level 1 | Equity securities
Assets
Total assets	64.8	64.8	3.5
Level 2
Assets
Total assets	1,602.8	1,602.8	1,185.8
Level 2 | Time deposits
Assets
Total assets	86.9	86.9	35.9
Level 2 | U.S. government and agency securities
Assets
Total assets	1,290.6	1,290.6	1,074.2
Level 2 | Corporate debt securities
Assets
Total assets	225.3	225.3	75.7
Level 3
Liabilities
Contingent consideration	13.4	13.4	13.4
Total liabilities	13.4	13.4	13.4
Balance
Assets
Total assets	1,921.0	1,921.0	1,436.9
Liabilities
Contingent consideration	13.4	13.4	13.4
Total liabilities	13.4	13.4	13.4
Balance | Money market funds
Assets
Total assets	253.4	253.4	247.6
Balance | Time deposits
Assets
Total assets	86.9	86.9	35.9
Balance | U.S. government and agency securities
Assets
Total assets	1,290.6	1,290.6	1,074.2
Balance | Corporate debt securities
Assets
Total assets	225.3	225.3	75.7
Balance | Equity securities
Assets
Total assets	$ 64.8	$ 64.8	$ 3.5