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]
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 Mar. 31, 2017	$ 20,674	1,265
Balance at beginning of period (As Previously Reported [Member]) at Dec. 31, 2016					1,560	(618)
Balance at beginning of period at Dec. 31, 2016			$ 0	$ 18,148	1,386	(444)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee benefit plans				(6)
Net income (loss)	296				296
Other comprehensive income (loss), net of tax	29					29
Balance at end of period at Mar. 31, 2017	19,409		0	18,142	1,682	(415)
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]					(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 Mar. 31, 2018	19,683	$ 1,265
Balance at beginning of period (As Previously Reported [Member]) at Dec. 31, 2017					1,130	(431)
Balance at beginning of period at Dec. 31, 2017	18,829		0	18,130	1,227	(518)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Employee benefit plans				(3)
Net income (loss)	(238)				(238)
Other comprehensive income (loss), net of tax	(180)					(180)
Balance at end of period at Mar. 31, 2018	$ 18,418		$ 0	$ 18,127	$ 989	$ (698)