SEGMENT INFORMATION - Financial Data (Details) $ in Thousands	3 Months Ended
	Mar. 28, 2020 USD ($) segment	Mar. 30, 2019 USD ($)	Dec. 28, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Number of reportable segments | segment	3
Total revenue	$ 367,190	$ 332,639
Loss from operations before taxes	(69,627)	(99,538)
Interest expense	27,818	30,105
Depreciation, depletion and amortization	51,201	54,807
Accretion	577	581
Loss on debt financings	0	14,565
Transaction costs	753	308
Non-cash compensation	4,905	5,906
Other	787	(146)
Total Adjusted EBITDA	16,414	6,588
Total purchases of property, plant and equipment	61,829	62,188
Total depreciation, depletion, amortization and accretion	51,778	55,388
Total assets	3,992,268		$ 4,067,556
West
Segment Reporting Information [Line Items]
Total revenue	196,225	181,945
East
Segment Reporting Information [Line Items]
Total revenue	133,040	113,388
Cement
Segment Reporting Information [Line Items]
Total revenue	37,925	37,306
Operating segment
Segment Reporting Information [Line Items]
Total purchases of property, plant and equipment	61,376	61,696
Total depreciation, depletion, amortization and accretion	50,789	54,436
Total assets	3,545,277		3,537,047
Operating segment | West
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	22,468	14,298
Total purchases of property, plant and equipment	18,896	30,375
Total depreciation, depletion, amortization and accretion	21,800	23,925
Total assets	1,368,542		1,379,684
Operating segment | East
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	9,573	3,242
Total purchases of property, plant and equipment	37,081	24,428
Total depreciation, depletion, amortization and accretion	21,096	20,211
Total assets	1,307,709		1,288,835
Operating segment | Cement
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	(7,561)	(2,587)
Total purchases of property, plant and equipment	5,399	6,893
Total depreciation, depletion, amortization and accretion	7,893	10,300
Total assets	869,026		868,528
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	(8,066)	(8,365)
Total purchases of property, plant and equipment	453	492
Total depreciation, depletion, amortization and accretion	989	952
Total assets	$ 446,991		530,509
Summit Materials, LLC
Segment Reporting Information [Line Items]
Number of operating segments | segment	3
Number of reportable segments | segment	3
Total revenue	$ 367,190	332,639
Loss from operations before taxes	(69,509)	(99,370)
Interest expense	27,700	29,937
Depreciation, depletion and amortization	51,201	54,807
Accretion	577	581
Loss on debt financings	0	14,565
Transaction costs	753	308
Non-cash compensation	4,905	5,906
Other	787	(146)
Total Adjusted EBITDA	16,414	6,588
Total purchases of property, plant and equipment	61,829	62,188
Total depreciation, depletion, amortization and accretion	51,778	55,388
Total assets	3,755,346		3,856,223
Summit Materials, LLC | West
Segment Reporting Information [Line Items]
Total revenue	196,225	181,945
Summit Materials, LLC | East
Segment Reporting Information [Line Items]
Total revenue	133,040	113,388
Summit Materials, LLC | Cement
Segment Reporting Information [Line Items]
Total revenue	37,925	37,306
Summit Materials, LLC | Operating segment
Segment Reporting Information [Line Items]
Total purchases of property, plant and equipment	61,376	61,696
Total depreciation, depletion, amortization and accretion	50,789	54,436
Total assets	3,545,277		3,537,047
Summit Materials, LLC | Operating segment | West
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	22,468	14,298
Total purchases of property, plant and equipment	18,896	30,375
Total depreciation, depletion, amortization and accretion	21,800	23,925
Total assets	1,368,542		1,379,684
Summit Materials, LLC | Operating segment | East
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	9,573	3,242
Total purchases of property, plant and equipment	37,081	24,428
Total depreciation, depletion, amortization and accretion	21,096	20,211
Total assets	1,307,709		1,288,835
Summit Materials, LLC | Operating segment | Cement
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	(7,561)	(2,587)
Total purchases of property, plant and equipment	5,399	6,893
Total depreciation, depletion, amortization and accretion	7,893	10,300
Total assets	869,026		868,528
Summit Materials, LLC | Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	(8,066)	(8,365)
Total purchases of property, plant and equipment	453	492
Total depreciation, depletion, amortization and accretion	989	$ 952
Total assets	$ 210,069		$ 319,176