Consolidated Statement of Changes in Equity - USD ($) $ in Millions	Total	Par Value	Capital in Excess of Par	Treasury Stock	Retained Earnings	Accum. Other Comprehensive Loss	Noncontrolling Interests
Beginning Balance at Dec. 31, 2018	$ 27,153	$ 6	$ 19,873	$ (15,023)	$ 20,489	$ (692)	$ 2,500
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	$ (89)
Beginning balance, common stock issued (in shares) at Dec. 31, 2018	645,692,000
Beginning balance, treasury stock (in shares) at Dec. 31, 2018	189,526,000
Beginning Balance at Dec. 31, 2018	$ 27,153	6	19,873	(15,023)	20,489	(692)	2,500
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	270				204		66
Other comprehensive income (loss)	72					72
Dividends paid on common stock	(364)				(364)
Repurchase of common stock	(344)			(344)
Benefit plan activity	2		4		(2)
Issuance of Phillips 66 Partners LP common units	21		2				19
Distributions to noncontrolling interests	(56)						(56)
Ending Balance at Mar. 31, 2019	$ 26,745	6	19,879	(15,367)	20,408	(709)	2,528
Beginning balance, common stock issued (in shares) at Dec. 31, 2018	645,692,000
Beginning balance, treasury stock (in shares) at Dec. 31, 2018	189,526,000
Stockholders' Equity, Shares [Roll Forward]
Repurchase of common stock (in shares)	3,639,000
Shares issued - share-based compensation (in shares)	1,024,000
Ending balance, common stock issued (in shares) at Mar. 31, 2019	646,716,000
Ending balance, treasury stock (in shares) at Mar. 31, 2019	193,165,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting changes	$ (9)				81	(89)	(1)
Beginning Balance at Dec. 31, 2019	27,169	6	20,301	(16,673)	22,064	(788)	2,259
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(2,427)				(2,496)		69
Other comprehensive income (loss)	(208)					(208)
Dividends paid on common stock	(396)				(396)
Repurchase of common stock	(443)			(443)
Benefit plan activity	2		4		(2)
Distributions to noncontrolling interests	(61)						(61)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	3				(2)	5
Ending Balance at Mar. 31, 2020	$ 23,639	$ 6	$ 20,305	$ (17,116)	$ 19,168	$ (991)	$ 2,267
Beginning balance, common stock issued (in shares) at Dec. 31, 2019	647,416,633
Beginning balance, treasury stock (in shares) at Dec. 31, 2019	206,390,806
Stockholders' Equity, Shares [Roll Forward]
Repurchase of common stock (in shares)	5,381,000
Shares issued - share-based compensation (in shares)	1,030,000
Ending balance, common stock issued (in shares) at Mar. 31, 2020	648,446,534
Ending balance, treasury stock (in shares) at Mar. 31, 2020	211,771,827