The interest rate used to adjust for the time value of money for the plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Expected Long-Term Rate of Return on Plan Assets
-URI http://asc.fasb.org/extlink&oid=6512046

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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Employee Benefit Plans - Weighted-Average Assumptions Used to Determine Benefit Obligations (Detail) - Summit Materials, LLC [Member]	12 Months Ended
	Jan. 02, 2016	Dec. 27, 2014	Dec. 28, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	7.30%	7.30%	7.50%
Pension Benefits [Member] \| Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.74%	3.50%
Pension Benefits [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.97%	3.65%
Healthcare & Life Ins [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate		3.52%
Healthcare & Life Ins [Member] \| Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.34%
Healthcare & Life Ins [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.80%