Summary of Significant Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Millions		Jan. 01, 2020	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities				$ 783
Operating lease ROU assets	[1]			719
Cumulative decrease in allowance for credit losses on loans				$ (637)	$ (541)	$ (681)	$ (1,017)
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liabilities			$ 780
Operating lease ROU assets			780
Cumulative effect of new accounting principle in period of adoption			$ 27
Subsequent Event [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ 82
Cumulative decrease in allowance for credit losses on loans		167
Cumulative increase in liability for off-balance sheet credit exposures		54
Subsequent Event [Member] | Student Loan [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		2
Subsequent Event [Member] | Commercial Loan [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative decrease in allowance for credit losses on loans		124
Subsequent Event [Member] | Consumer Loans [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative decrease in allowance for credit losses on loans		$ 43

[1] Reported net of level lease adjustments, exit cost obligations and accumulated impairment losses.