Fair Value Measurements (Details) - Recurring fair value measurements - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Level 1
Assets
Total assets	$ 579.0	$ 534.4
Level 1 | Money market funds
Assets
Total assets	579.0	534.4
Level 2
Assets
Total assets	370.7	30.1
Level 2 | Time deposits
Assets
Total assets	25.1
Level 2 | U.S. government and agency securities
Assets
Total assets	341.7	19.3
Level 2 | Corporate debt securities
Assets
Total assets	3.9	10.8
Level 3
Liabilities
Contingent consideration	10.4	10.4
Total liabilities	10.4	10.4
Balance
Assets
Total assets	949.7	564.5
Liabilities
Contingent consideration	10.4	10.4
Total liabilities	10.4	10.4
Balance | Money market funds
Assets
Total assets	579.0	534.4
Balance | Time deposits
Assets
Total assets	25.1
Balance | U.S. government and agency securities
Assets
Total assets	341.7	19.3
Balance | Corporate debt securities
Assets
Total assets	$ 3.9	$ 10.8