Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 31, 2018	Jan. 17, 2017
Business Acquisition [Line Items]
Loss on early debt extinguishment	$ (14)	$ 0	$ 0
Debt guarantee arrangements, fair value	6,000	5,000
Contingent consideration liability, fair value		$ 15
Elan Financial Services
Business Acquisition [Line Items]
Contingent consideration liability, fair value	12			$ 12
Online Banking Solutions, Inc.
Business Acquisition [Line Items]
Contingent consideration liability, fair value	5				$ 15
Non-cash fair value adjustment decrease on contingent consideration	10
Variable-Rate Revolving Credit Facilities and Variable-Rate Term Loan Facilities Due March 2023
Business Acquisition [Line Items]
Debt guarantee arrangements, fair value	$ 29