Fair Value Measurements (Details) - Recurring fair value measurements - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Level 1
Assets
Total assets	$ 641.5	$ 534.4
Level 1 | Money market funds
Assets
Total assets	641.5	534.4
Level 2
Assets
Total assets	509.8	30.1
Level 2 | Time deposits
Assets
Total assets	25.2
Level 2 | U.S. government and agency securities
Assets
Total assets	480.5	19.3
Level 2 | Corporate debt securities
Assets
Total assets	4.1	10.8
Level 3
Liabilities
Contingent consideration	10.4	10.4
Total liabilities	10.4	10.4
Balance
Assets
Total assets	1,151.3	564.5
Liabilities
Contingent consideration	10.4	10.4
Total liabilities	10.4	10.4
Balance | Money market funds
Assets
Total assets	641.5	534.4
Balance | Time deposits
Assets
Total assets	25.2
Balance | U.S. government and agency securities
Assets
Total assets	480.5	19.3
Balance | Corporate debt securities
Assets
Total assets	$ 4.1	$ 10.8