Fair Value Measurements (Details) - Recurring fair value measurements - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Level 1
Assets
Total assets	$ 444.1	$ 251.1
Level 1 | Money market funds
Assets
Total assets	437.6	247.6
Level 1 | Equity securities
Assets
Total assets	6.5	3.5
Level 2
Assets
Total assets	1,261.2	1,185.8
Level 2 | Time deposits
Assets
Total assets	38.4	35.9
Level 2 | U.S. government and agency securities
Assets
Total assets	1,111.1	1,074.2
Level 2 | Corporate debt securities
Assets
Total assets	111.7	75.7
Level 3
Liabilities
Contingent consideration	13.4	13.4
Total liabilities	13.4	13.4
Balance
Assets
Total assets	1,705.3	1,436.9
Liabilities
Contingent consideration	13.4	13.4
Total liabilities	13.4	13.4
Balance | Money market funds
Assets
Total assets	437.6	247.6
Balance | Time deposits
Assets
Total assets	38.4	35.9
Balance | U.S. government and agency securities
Assets
Total assets	1,111.1	1,074.2
Balance | Corporate debt securities
Assets
Total assets	111.7	75.7
Balance | Equity securities
Assets
Total assets	$ 6.5	$ 3.5