Basis of Presentation -Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in assets	$ 841
Increase in liabilities	968
Decrease in shareholders' equity	32,303	$ 31,764	$ 30,178	$ 30,007	$ 29,832	$ 29,695
Estimated Impact of Lease Accounting Standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in assets	860
Increase in liabilities	997
Decrease in shareholders' equity	$ 40
Estimated CECL impact on Allowance for Credit Losses | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
financing receivable, allowance for credit loss, period increase percent	30.00%
Estimated CECL impact on Allowance for Credit Losses | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
financing receivable, allowance for credit loss, period increase percent	50.00%