Stockholders' Equity/Members' Interest - Change in Ownership (Details) - shares	12 Months Ended
Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Increase (Decrease) In Stockholders' Equity [Rollforward]
Stock option exercises (in shares)		1,065,446
Summit Materials Incorporated And Summit Holdings LP
Increase (Decrease) In Stockholders' Equity [Rollforward]
Beginning balance stockholders' equity (in shares)	116,559,042	115,094,445
Exchanges during period (in shares)	0	0
Stock option exercises (in shares)	54,517
Other equity transactions (in shares)	650,206	399,151
Ending balance stockholders' equity (in shares)	117,263,765	116,559,042	115,094,445
LP Units \| Summit Holdings LP
Increase (Decrease) In Stockholders' Equity [Rollforward]
Number of LP Units outstanding (in shares)	3,249,657	3,435,518
Number of LP Units exchanged	(376,487)	(185,861)
Number of LP Units outstanding (in shares)	2,873,170	3,249,657	3,435,518
Common Class A
Increase (Decrease) In Stockholders' Equity [Rollforward]
Beginning balance stockholders' equity (in shares)	113,309,385	111,658,927
Exchanges during period (in shares)	376,487	185,861
Stock option exercises (in shares)	54,517	1,065,446
Other equity transactions (in shares)	650,206	399,151
Ending balance stockholders' equity (in shares)	114,390,595	113,309,385	111,658,927
Summit Materials Inc
Increase (Decrease) In Stockholders' Equity [Rollforward]
Summit Inc. Ownership Percentage	97.50%	97.20%	97.00%

The number of limited partner units exchanged.

No definition available.

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- Definition

Represents other stockholder equity activity, in shares.

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No definition available.

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- Definition

Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

+ References

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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The number of limited partner units outstanding.

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- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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- Definition

The number of shares issued during the period upon the conversion of units. An example of a convertible unit is an umbrella partnership real estate investment trust unit (UPREIT unit).

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Number of share options (or share units) exercised during the current period.

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