Basis of Presentation and Signficant Accounting Policies Basis of Presentation and Significant Acconting Policies (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease Liabilities		$ 699
Cumulative effect of new accounting principle in period of adoption				$ 26
Customer funds and cash and cash equivalents on deposit for regulatory requirements		$ 263		$ 222
Approximate percentage of revenues subject to new revenue guidance under ASC 606		6.00%
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Right-of-Use Asset			$ 637
Operating Lease Liabilities			728
Cumulative effect of new accounting principle in period of adoption			$ 26
Accounting Standards Update 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2018-02	$ 91
Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption					$ 320