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 Sep. 30, 2017	$ 20,998	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				(19)
Net income (loss)	594				594
Other comprehensive income (loss), net of tax	106					106
Balance at end of period at Sep. 30, 2017	19,733		0	18,129	1,942	(338)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared on preferred stock					(38)
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 Sep. 30, 2018	20,000	$ 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				6
Net income (loss)	208				208
Other comprehensive income (loss), net of tax	(280)					(280)
Balance at end of period at Sep. 30, 2018	$ 18,735		$ 0	$ 18,136	1,397	$ (798)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cash dividends declared on preferred stock					$ (38)