Supplementary Data (Unaudited) - Supplemental Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 30, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Jan. 02, 2016	Dec. 30, 2017	Dec. 31, 2016	Jan. 02, 2016
Condensed Income Statements, Captions [Line Items]
Net revenue	$ 440,610	$ 574,387	$ 478,368	$ 259,044	$ 387,389	$ 480,210	$ 412,636	$ 208,039		$ 1,752,409	$ 1,488,274	$ 1,289,966
Operating income (loss)	57,306	113,911	82,444	(32,784)	48,761	88,410	46,948	(29,457)		220,877	154,662	135,024
Net income (loss)	44,510	84,287	52,088	(55,108)	6,049	61,106	21,505	(42,534)	$ 44,789	125,777	46,126	1,484
Net income (loss) attributable to Summit Inc.	$ 43,010	$ 81,264	$ 50,000	$ (52,444)	$ (290)	$ 44,820	$ 13,371	$ (21,118)		$ 121,830	$ 36,783	27,718
Basic earnings per share attributable to Summit Inc.	$ 0.39	$ 0.74	$ 0.46	$ (0.49)	$ 0.00	$ 0.59	$ 0.21	$ (0.40)
Diluted earnings per share attributable to Summit Inc.	$ 0.38	$ 0.73	$ 0.46	$ (0.49)	$ 0.00	$ 0.59	$ 0.20	$ (0.40)
Retroactive application of common stock issued as dividends (in shares)										1,521,056	1,135,962
Summit Materials, LLC
Condensed Income Statements, Captions [Line Items]
Net revenue	$ 440,610	$ 574,387	$ 478,368	$ 259,044	$ 387,389	$ 480,210	$ 412,636	$ 208,039		$ 1,752,409	$ 1,488,274	1,289,966
Operating income (loss)	57,306	113,911	82,444	(32,784)	48,761	88,410	46,948	(29,457)		220,877	154,662	135,024
Net income (loss)	$ 52,435	$ 82,633	$ 53,827	$ (54,854)	$ 21,211	$ 61,360	$ 21,759	$ (42,243)		134,041	62,087	2,356
Net income (loss) attributable to Summit Inc.										$ 134,068	$ 62,071	$ 4,182