Summary of Organization and Significant Accounting Policies - General Information and Equity Offerings (Details) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended						9 Months Ended
	Jan. 10, 2017 USD ($) item $ / shares shares	Dec. 28, 2016 shares	Oct. 31, 2017 shares	Sep. 30, 2017 shares	Jul. 01, 2017 shares	Apr. 01, 2017 shares	Oct. 01, 2016 shares	Jul. 02, 2016 shares	Apr. 02, 2016 shares	Sep. 30, 2017 USD ($) segment facility shares	Dec. 31, 2016 shares	Jan. 02, 2016 shares
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Number of operating segments | segment										3
Equity Offering
Proceeds from sale of common stock, net of underwriting discounts | $										$ 237,600
Principles of Consolidation
Ownership Percentage				96.50%	96.30%	95.60%	75.80%	63.30%	50.30%	96.50%	94.90%	50.30%
Cement plant
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Number of plants | facility										2
Ohio Valley Asphalt
Principles of Consolidation
Noncontrolling interest elimination (as a percent)				20.00%						20.00%
Minority interest percentage purchased			20.00%
Summit Materials, LLC
Summary Of Significant Accounting Policies And Recent Accounting Pronouncements
Number of operating segments | segment										3
Summit Materials, LLC | Ohio Valley Asphalt
Principles of Consolidation
Noncontrolling interest elimination (as a percent)				20.00%						20.00%
Minority interest percentage purchased			20.00%
Summit Holdings LP
Principles of Consolidation
Voting power (as a percent)				100.00%						100.00%
Noncontrolling interest elimination (as a percent)				3.50%						3.50%	5.10%
Summit Materials, Inc. and Summit Holdings, LP
Equity Offering
Common stock issued (in shares)						10,000,000			1,038
Principles of Consolidation
Beginning Balance (in shares)			112,361,719	111,629,117	111,318,942	101,184,519	101,164,749	101,158,499	101,157,461	101,184,519
Issuance of Shares (in shares)						10,000,000			1,038
Other (in shares)				732,602	310,175	134,423	833	6,250
Ending Balance (in shares)				112,361,719	111,629,117	111,318,942	101,165,582	101,164,749	101,158,499	112,361,719
Common Class A
Equity Offering
Common stock issued (in shares)						10,000,000			1,038
Principles of Consolidation
Beginning Balance (in shares)			108,465,688	107,491,979	106,403,740	96,033,222	64,066,678	50,882,674	50,881,636	96,033,222
Issuance of Shares (in shares)						10,000,000			1,038
Stock Dividend		1,135,692
Exchanges during period				241,107	778,064	236,095	12,634,500	13,177,754
Other (in shares)				732,602	310,175	134,423	833	6,250
Ending Balance (in shares)				108,465,688	107,491,979	106,403,740	76,702,011	64,066,678	50,882,674	108,465,688
Common Class B
Principles of Consolidation
Beginning Balance (in shares)			100			100				100
Ending Balance (in shares)				100						100
Follow on Public Offering
Equity Offering
Proceeds from sale of common stock, net of underwriting discounts | $	$ 237,600
Common stock issued (in shares)	10,000,000
Offering price (in dollars per share) | $ / shares	$ 24.05
Cash paid for acquisitions | $	$ 110,000
Number of companies acquired | item	2
Principles of Consolidation
Issuance of Shares (in shares)	10,000,000
LP Units | Summit Holdings LP
Principles of Consolidation
Number of LP Units outstanding				3,896,031	4,137,138	4,915,202	24,463,571	37,098,071	50,275,825	3,896,031	5,151,297	50,275,825
Number of LP Units exchanged				(241,107)	778,064	236,095	(12,634,500)	13,177,754