Goodwill and Purchased Intangible Assets - Goodwill Rollforward (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) reportableSegment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Goodwill [Roll Forward]
Goodwill, beginning of period		$ 3,337,100,000	$ 3,108,800,000
Additions due to business combinations		332,500,000	228,300,000
Goodwill, end of period	$ 3,669,600,000	3,669,600,000	3,337,100,000	$ 3,108,800,000
Number of reporting units | reportableSegment	3
Goodwill impairment		$ 0	$ 0	$ 0