CONSOLIDATED STATEMENT OF CHANGES IN EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	As Previously Reported [Member] Retained Earnings [Member]	As Previously Reported [Member] Accumulated Other Comprehensive Loss [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01, Equity Investments Previously Classified as Available-for-sale [Member]					$ 0	$ 0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01, Equity Investments Previously Measured at Cost [Member]					0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2018-02 [Member]					0	0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01, Fair Value Option [Member]					(174)	174
Balance at beginning of period at Dec. 31, 2016		$ 1,265
Balance at end of period at Jun. 30, 2017	$ 20,937	1,265
Balance at beginning of period at Dec. 31, 2016			$ 0	$ 18,148	1,386	(444)	$ 1,560	$ (618)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee benefit plans				(6)
Net income (loss)	500				500
Other comprehensive income (loss), net of tax	127					127
Balance at end of period at Jun. 30, 2017	19,672		0	18,142	1,847	(317)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared on preferred stock					(39)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01, Equity Investments Previously Classified as Available-for-sale [Member]					(4)	4
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01, Equity Investments Previously Measured at Cost [Member]					10
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2018-02 [Member]					91	(91)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01, Fair Value Option [Member]					0	0
Balance at beginning of period at Dec. 31, 2017	20,094	1,265
Balance at end of period at Jun. 30, 2018	19,985	$ 1,265
Balance at beginning of period at Dec. 31, 2017	18,829		0	18,130	1,227	(518)	$ 1,130	$ (431)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee benefit plans				5
Net income (loss)	31				31
Other comprehensive income (loss), net of tax	(118)					(118)
Balance at end of period at Jun. 30, 2018	$ 18,720		$ 0	$ 18,135	1,221	$ (636)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared on preferred stock					$ (37)