Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic
Net income attributable to Phillips 66									$ 3,076	$ 5,595	$ 5,106
Income allocated to participating securities									(6)	(6)	(6)
Net income available to common stockholders									$ 3,070	$ 5,589	$ 5,100
Weighted-average common shares outstanding (in shares)									448,787	467,483	511,268
Effect of share-based compensation (in shares)									2,577	3,225	3,822
Weighted-average commons shares outstanding (in shares)									451,364	470,708	515,090
Earnings Per Share of Common Stock (in dollars per share)	$ 1.65	$ 1.58	$ 3.13	$ 0.44	$ 4.85	$ 3.20	$ 2.86	$ 1.07	$ 6.80	$ 11.87	$ 9.90
Diluted
Net income attributable to Phillips 66									$ 3,076	$ 5,595	$ 5,106
Income allocated to participating securities									(2)	0	0
Net income available to common stockholders									$ 3,074	$ 5,595	$ 5,106
Weighted-average commons shares outstanding (in shares)									451,364	470,708	515,090
Effect of share-based compensation (in shares)									2,524	3,339	3,418
Weighted-average common shares outstanding—EPS (in shares)									453,888	474,047	518,508
Earnings Per Share of Common Stock (in dollars per share)	$ 1.64	$ 1.58	$ 3.12	$ 0.44	$ 4.82	$ 3.18	$ 2.84	$ 1.07	$ 6.77	$ 11.80	$ 9.85