Consolidated Statement of Changes in Equity - USD ($) $ in Millions	Total	Cumulative effect of accounting changes	Common Stock	Capital in Excess of Par	Treasury Stock	Retained Earnings	Retained Earnings Cumulative effect of accounting changes	Accum. Other Comprehensive Loss	Accum. Other Comprehensive Loss Cumulative effect of accounting changes	Noncontrolling Interests	Noncontrolling Interests Cumulative effect of accounting changes
Beginning Balance at Dec. 31, 2017	$ 27,428	$ 49	$ 6	$ 19,768	$ (10,378)	$ 16,306	$ 36	$ (617)		$ 2,343	$ 13
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	5,873					5,595				278
Other comprehensive loss	(75)							(75)
Dividends paid on common stock	(1,436)					(1,436)
Repurchase of common stock	(4,645)				(4,645)
Benefit plan activity	51			63		(12)
Issuance of Phillips 66 Partners LP common units	115			42						73
Distributions to noncontrolling interests	(207)									(207)
Ending Balance at Dec. 31, 2018	$ 27,153	$ (9)	6	19,873	(15,023)	20,489	$ 81	(692)	$ (89)	2,500	$ (1)
Beginning Balance, Common shares (in shares) at Dec. 31, 2017	643,835,464
Beginning Balance, Treasury stock (in shares) at Dec. 31, 2017	141,565,145
Stockholders' Equity, Shares [Roll Forward]
Repurchase of common stock (in shares)	47,961,186
Shares issued—share-based compensation (in shares)	1,856,297
Ending Balance, Common shares (in shares) at Dec. 31, 2018	645,691,761
Ending Balance, Treasury stock (in shares) at Dec. 31, 2018	189,526,331
Dividends, Common Stock [Abstract]
Dividends Paid Per Share of Common Stock (in usd per share)	$ 3.10
Net income	$ 3,377					3,076				301
Other comprehensive loss	(7)							(7)
Dividends paid on common stock	(1,570)					(1,570)
Repurchase of common stock	(1,650)				(1,650)
Benefit plan activity	73			85		(12)
Issuance of Phillips 66 Partners LP common units	141			68		0				73
Impacts from Phillips 66 Partners LP GP/IDR restructuring transaction	(98)			275						(373)
Distributions to noncontrolling interests	(241)									(241)
Ending Balance at Dec. 31, 2019	$ 27,169		6	20,301	(16,673)	22,064		(788)		2,259
Stockholders' Equity, Shares [Roll Forward]
Repurchase of common stock (in shares)	16,864,475
Shares issued—share-based compensation (in shares)	1,724,872
Ending Balance, Common shares (in shares) at Dec. 31, 2019	647,416,633
Ending Balance, Treasury stock (in shares) at Dec. 31, 2019	206,390,806
Dividends, Common Stock [Abstract]
Dividends Paid Per Share of Common Stock (in usd per share)	$ 3.50
Net income	$ (3,714)					(3,975)				261
Other comprehensive loss	(6)							(6)
Dividends paid on common stock	(1,575)					(1,575)
Repurchase of common stock	(443)				(443)
Benefit plan activity	68			80		(12)
Transfer of equity interest	307			2						305
Net distributions to noncontrolling interests	(285)									(285)
Other	2					(2)		5		(1)
Ending Balance at Dec. 31, 2020	$ 21,523		$ 6	$ 20,383	$ (17,116)	$ 16,500		$ (789)		$ 2,539
Stockholders' Equity, Shares [Roll Forward]
Repurchase of common stock (in shares)	5,381,021
Shares issued—share-based compensation (in shares)	1,226,590
Ending Balance, Common shares (in shares) at Dec. 31, 2020	648,643,223
Ending Balance, Treasury stock (in shares) at Dec. 31, 2020	211,771,827
Dividends, Common Stock [Abstract]
Dividends Paid Per Share of Common Stock (in usd per share)	$ 3.60