Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

The cumulative amount of the reporting entity's undistributed earnings or deficit.

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Significant Accounting Policies - Consolidated Balance Sheet as Adjusted (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
3M Company shareholders’ equity:
Retained earnings	$ 45,361	$ 43,821
Accumulated other comprehensive income (loss)	$ (7,620)	(7,721)
Under Prior Method
3M Company shareholders’ equity:
Retained earnings		43,761
Accumulated other comprehensive income (loss)		$ (7,661)
Cumulative Effect, Period of Adoption, Adjustment
3M Company shareholders’ equity:
Retained earnings			$ (5)
Accumulated other comprehensive income (loss)			$ (5)