Summary of Significant Accounting Policies - Goodwill (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Goodwill impairment	$ 0
Accumulated impairment loss	0	$ 0
Goodwill [Roll Forward]
Goodwill, beginning balance		5,373,000,000	$ 5,200,000,000
Acquired goodwill		217,000,000	173,000,000
Disposed goodwill		(3,000,000)
Foreign currency adjustments		3,000,000
Goodwill, ending balance	5,590,000,000	5,590,000,000	5,373,000,000
Payments
Goodwill [Roll Forward]
Goodwill, beginning balance		3,610,000,000	3,437,000,000
Acquired goodwill		146,000,000	173,000,000
Disposed goodwill		0
Foreign currency adjustments		1,000,000
Goodwill, ending balance	3,757,000,000	3,757,000,000	3,610,000,000
Financial
Goodwill [Roll Forward]
Goodwill, beginning balance		1,763,000,000	1,763,000,000
Acquired goodwill		71,000,000	0
Disposed goodwill		(3,000,000)
Foreign currency adjustments		2,000,000
Goodwill, ending balance	$ 1,833,000,000	$ 1,833,000,000	$ 1,763,000,000