Summary of Significant Accounting Policies - Consolidated Statement of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	$ 4,847	$ 2,160	$ 3,554
Pension and other postretirement measurement gains and losses	114	468	(217)
Income taxes	487	(380)	(52)
Other operating assets and liabilities	50	(855)	13
Net cash provided by operating activities	5,129	5,175	4,314
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	4,811	2,252	3,189
Pension and other postretirement measurement gains and losses	0	0	0
Income taxes	477	(356)	(245)
Other operating assets and liabilities	(110)	(1,207)	(328)
Net cash provided by operating activities	5,129	5,175	4,314
Impact of change | Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	36	(92)	365
Pension and other postretirement measurement gains and losses	114	468	(217)
Income taxes	10	(24)	193
Other operating assets and liabilities	160	352	341
Net cash provided by operating activities	$ 0	$ 0	$ 0