Segment Information - Financial Data (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Segment Reporting Information [Line Items]
Revenue	$ 2,332,451	$ 2,222,140	$ 2,101,002
Income from operations before taxes	129,055	78,224	96,077
Interest expense	103,595	116,509	116,548
Depreciation, depletion and amortization	218,682	214,886	203,305
Accretion	2,638	2,216	1,605
Loss on debt financings	4,064	14,565	149
Tax receivable agreement (benefit) expense	(7,559)	16,237	(22,684)
Gain on sale of business	0	0	(12,108)
Transaction costs	2,747	2,222	4,238
Non-cash compensation	28,857	20,403	25,378
Other	2,957	(3,800)	(6,247)
Total Adjusted EBITDA	485,036	461,462	406,261
Total purchases of property, plant and equipment	177,249	177,495	220,685
Total depreciation, depletion, amortization and accretion	221,320	217,102	204,910
Total assets	4,308,011	4,067,556	3,857,641
Operating segment
Segment Reporting Information [Line Items]
Total purchases of property, plant and equipment	175,099	174,982	213,077
Total depreciation, depletion, amortization and accretion	217,338	213,106	202,288
Total assets	3,657,959	3,537,047	3,501,727
Corporate and other
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	(41,247)	(34,565)	(32,164)
Total purchases of property, plant and equipment	2,150	2,513	7,608
Total depreciation, depletion, amortization and accretion	3,982	3,996	2,622
Total assets	650,052	530,509	355,914
West
Segment Reporting Information [Line Items]
Revenue	1,262,196	1,122,338	1,117,066
West | Operating segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	271,052	204,964	188,999
Total purchases of property, plant and equipment	67,500	71,397	120,657
Total depreciation, depletion, amortization and accretion	93,866	93,256	91,794
Total assets	1,503,382	1,379,684	1,370,501
East
Segment Reporting Information [Line Items]
Revenue	799,633	809,098	703,147
East | Operating segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	162,275	187,625	138,032
Total purchases of property, plant and equipment	92,528	77,894	64,384
Total depreciation, depletion, amortization and accretion	86,205	81,403	75,433
Total assets	1,303,742	1,288,835	1,253,640
Cement
Segment Reporting Information [Line Items]
Revenue	270,622	290,704	280,789
Cement | Operating segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	92,956	103,438	111,394
Total purchases of property, plant and equipment	15,071	25,691	28,036
Total depreciation, depletion, amortization and accretion	37,267	38,447	35,061
Total assets	850,835	868,528	877,586
Summit Materials, LLC
Segment Reporting Information [Line Items]
Revenue	2,332,451	2,222,140	2,101,002
Income from operations before taxes	121,800	94,982	74,110
Interest expense	103,291	115,988	115,831
Depreciation, depletion and amortization	218,682	214,886	203,305
Accretion	2,638	2,216	1,605
Loss on debt financings	4,064	14,565	149
Gain on sale of business	0	0	(12,108)
Transaction costs	2,747	2,222	4,238
Non-cash compensation	28,857	20,403	25,378
Other	2,957	(3,800)	(6,247)
Total Adjusted EBITDA	485,036	461,462	406,261
Total purchases of property, plant and equipment	177,249	177,495	220,685
Total depreciation, depletion, amortization and accretion	221,320	217,102	204,910
Total assets	4,077,134	3,856,223	3,633,244
Summit Materials, LLC | Operating segment
Segment Reporting Information [Line Items]
Total purchases of property, plant and equipment	175,099	174,982	213,077
Total depreciation, depletion, amortization and accretion	217,338	213,106	202,288
Total assets	3,657,959	3,537,047	3,501,727
Summit Materials, LLC | Corporate and other
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	(41,247)	(34,565)	(32,164)
Total purchases of property, plant and equipment	2,150	2,513	7,608
Total depreciation, depletion, amortization and accretion	3,982	3,996	2,622
Total assets	419,175	319,176	131,517
Summit Materials, LLC | West
Segment Reporting Information [Line Items]
Revenue	1,262,196	1,122,338	1,117,066
Summit Materials, LLC | West | Operating segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	271,052	204,964	188,999
Total purchases of property, plant and equipment	67,500	71,397	120,657
Total depreciation, depletion, amortization and accretion	93,866	93,256	91,794
Total assets	1,503,382	1,379,684	1,370,501
Summit Materials, LLC | East
Segment Reporting Information [Line Items]
Revenue	799,633	809,098	703,147
Summit Materials, LLC | East | Operating segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	162,275	187,625	138,032
Total purchases of property, plant and equipment	92,528	77,894	64,384
Total depreciation, depletion, amortization and accretion	86,205	81,403	75,433
Total assets	1,303,742	1,288,835	1,253,640
Summit Materials, LLC | Cement
Segment Reporting Information [Line Items]
Revenue	270,622	290,704	280,789
Summit Materials, LLC | Cement | Operating segment
Segment Reporting Information [Line Items]
Total Adjusted EBITDA	92,956	103,438	111,394
Total purchases of property, plant and equipment	15,071	25,691	28,036
Total depreciation, depletion, amortization and accretion	37,267	38,447	35,061
Total assets	$ 850,835	$ 868,528	$ 877,586