Summary of Significant Accounting Policies and New Accounting Pronouncements Summary o Significant Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Millions		Jan. 01, 2018	Jan. 01, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption	[1]			$ 174	$ 0	$ 0
Accounting Standards Update 2016-18 [Member] | Subsequent Event [Member] | Year Ended December 31, 2017 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in cash provided by investing activities		$ (210)
Accounting Standards Update 2016-18 [Member] | Subsequent Event [Member] | Year ended December 31, 2016 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in cash provided by investing activities		951
Retained Earnings [Member] | 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			$ (174)
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		8
Retained Earnings [Member] | Accounting Standards Update 2018-02 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		91
Accumulated Other Comprehensive Loss [Member] | 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			$ 174
Accumulated Other Comprehensive Loss [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		4
Accumulated Other Comprehensive Loss [Member] | Accounting Standards Update 2018-02 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		(91)
Equity Investments at Fair Value [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		8
To Retained Earnings from AOCI [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		(4)
Trading Assets [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		(12)
To Other Assets from Trading Assets [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		12
Securities available-for-sale [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		(177)
To Other Assets From Securities Available-for-sale [Member] | Accounting Standards Update 2016-01 [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ 177

[1] See Note 2, "Summary of Significant Accounting Polices and New Accounting Pronouncements," for additional discussion.