Basis of Presentation and Accounting Policies Basis of Presentation and Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jan. 01, 2018	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of changes to retained earnings as a result of adoption of ASU	$ 7,786	$ 8,034	$ 7,786	$ 8,034	$ 7,963		$ 7,796
Proceeds from settlement of BOLI policies			4	8
Remeasurement of the Corporation's deferred tax balance			99		$ 107
Deferred tax balance downward revision to the estimated impact			8
Accounting Standards Codification (ASC) Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in card fees due to change in presentation	34		105
Decrease in service charges on deposit accounts due to change in presentation	2		4
Decrease in processing fee expense due to change in presentation	36		109
Treasury management fee transition adjustment to retained earnings						$ 14
Cumulative effect of changes to retained earnings as a result of adoption of ASU						$ 2
Network costs to be presented net of card fee revenue	34	29	105	80
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from settlement of BOLI policies				8
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other components of net benefit cost to be reclassified		$ 12		$ 36
Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Investment Transferred from Available-for-sale to Equity Method, before Tax			1
Recognition of additional unrealized losses from the transfer of Held-to-maturity to Available-for-sale securities			11
Minimum | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Preliminary right-of-use asset value from the implementation of ASU 2016-02	450		450
Preliminary lease obligation liability from the implementation of ASU 2016-02	450		450
Maximum | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Preliminary right-of-use asset value from the implementation of ASU 2016-02	550		550
Preliminary lease obligation liability from the implementation of ASU 2016-02	$ 550		$ 550