SEGMENT INFORMATION - Financial Data (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 28, 2019 USD ($)	Sep. 29, 2018 USD ($)	Sep. 28, 2019 USD ($) segment	Sep. 29, 2018 USD ($)	Dec. 29, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment				3
Number of reportable segments | segment				3
Total revenue		$ 732,082	$ 694,661	$ 1,665,612	$ 1,609,972
Income from operations before taxes		103,839	94,757	58,998	71,206
Interest expense		28,917	28,889	88,423	86,616
Depreciation, depletion and amortization		54,575	53,494	162,417	149,439
Accretion		552	480	1,723	1,224
Loss on debt financings		0	0	14,565	149
Gain on sale of business		0	(12,108)	0	(12,108)
Transaction costs		751	1,260	1,449	3,817
Non-cash compensation		4,819	5,643	15,424	19,833
Other		(136)	(409)	(2,628)	(7,316)
Total Adjusted EBITDA		193,317	172,006	340,371	312,860
Total purchases of property, plant and equipment				139,762	183,752
Total depreciation, depletion, amortization and accretion		55,127	53,974	164,140	150,663
Total assets		4,042,114		4,042,114		$ 3,857,641
West
Segment Reporting Information [Line Items]
Total revenue		366,504	367,912	848,661	871,338
East
Segment Reporting Information [Line Items]
Total revenue		266,587	232,777	596,107	525,270
Cement
Segment Reporting Information [Line Items]
Total revenue		98,991	93,972	220,844	213,364
Operating segment
Segment Reporting Information [Line Items]
Total purchases of property, plant and equipment				138,596	177,806
Total depreciation, depletion, amortization and accretion		54,086	53,400	161,155	148,744
Total assets		3,653,397		3,653,397		3,501,727
Operating segment | West
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		81,936	73,916	151,054	151,316
Total purchases of property, plant and equipment				61,679	104,217
Total depreciation, depletion, amortization and accretion		23,307	23,289	70,156	68,029
Total assets		1,440,010		1,440,010		1,370,501
Operating segment | East
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		76,825	58,305	134,479	100,497
Total purchases of property, plant and equipment				61,830	51,968
Total depreciation, depletion, amortization and accretion		19,668	19,429	59,719	54,982
Total assets		1,334,247		1,334,247		1,253,640
Operating segment | Cement
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		42,683	44,299	75,537	82,626
Total purchases of property, plant and equipment				15,087	21,621
Total depreciation, depletion, amortization and accretion		11,111	10,682	31,280	25,733
Total assets		879,140		879,140		877,586
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		(8,127)	(4,514)	(20,699)	(21,579)
Total purchases of property, plant and equipment				1,166	5,946
Total depreciation, depletion, amortization and accretion		1,041	574	2,985	1,919
Total assets		388,717		$ 388,717		355,914
Summit Materials, LLC
Segment Reporting Information [Line Items]
Number of operating segments | segment				3
Number of reportable segments | segment				3
Total revenue		732,082	694,661	$ 1,665,612	1,609,972
Income from operations before taxes		103,956	94,926	59,401	71,756
Interest expense		28,800	28,720	88,020	86,066
Depreciation, depletion and amortization		54,575	53,494	162,417	149,439
Accretion		552	480	1,723	1,224
Loss on debt financings		0	0	14,565	149
Gain on sale of business		0	(12,108)	0	(12,108)
Transaction costs		751	1,260	1,449	3,817
Non-cash compensation		4,819	5,643	15,424	19,833
Other		(136)	(409)	(2,628)	(7,316)
Total Adjusted EBITDA		193,317	172,006	340,371	312,860
Total purchases of property, plant and equipment				139,762	183,752
Total depreciation, depletion, amortization and accretion		55,127	53,974	164,140	150,663
Total assets		3,849,482		3,849,482		3,633,244
Summit Materials, LLC | West
Segment Reporting Information [Line Items]
Total revenue		366,504	367,912	848,661	871,338
Gain on sale of business	$ (12,100)
Summit Materials, LLC | East
Segment Reporting Information [Line Items]
Total revenue		266,587	232,777	596,107	525,270
Summit Materials, LLC | Cement
Segment Reporting Information [Line Items]
Total revenue		98,991	93,972	220,844	213,364
Summit Materials, LLC | Operating segment
Segment Reporting Information [Line Items]
Total purchases of property, plant and equipment				138,596	177,806
Total depreciation, depletion, amortization and accretion		54,086	53,400	161,155	148,744
Total assets		3,653,397		3,653,397		3,501,727
Summit Materials, LLC | Operating segment | West
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		81,936	73,916	151,054	151,316
Total purchases of property, plant and equipment				61,679	104,217
Total depreciation, depletion, amortization and accretion		23,307	23,289	70,156	68,029
Total assets		1,440,010		1,440,010		1,370,501
Summit Materials, LLC | Operating segment | East
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		76,825	58,305	134,479	100,497
Total purchases of property, plant and equipment				61,830	51,968
Total depreciation, depletion, amortization and accretion		19,668	19,429	59,719	54,982
Total assets		1,334,247		1,334,247		1,253,640
Summit Materials, LLC | Operating segment | Cement
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		42,683	44,299	75,537	82,626
Total purchases of property, plant and equipment				15,087	21,621
Total depreciation, depletion, amortization and accretion		11,111	10,682	31,280	25,733
Total assets		879,140		879,140		877,586
Summit Materials, LLC | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA		(8,127)	(4,514)	(20,699)	(21,579)
Total purchases of property, plant and equipment				1,166	5,946
Total depreciation, depletion, amortization and accretion		1,041	$ 574	2,985	$ 1,919
Total assets		$ 196,085		$ 196,085		$ 131,517