Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Class of Stock [Line Items]
Total cost	$ 480.1	$ 959.6
Remaining authorized repurchase amount	1,571.5
Common Class A and Common Class C [Member]
Class of Stock [Line Items]
Total cost	$ 496.3
Common Class A [Member]
Class of Stock [Line Items]
Shares repurchased (in shares)	2,258,800
Average price paid per share (in dollars per share)	$ 32.44
Common Class C [Member]
Class of Stock [Line Items]
Shares repurchased (in shares)	12,551,500
Average price paid per share (in dollars per share)	$ 33.70