Stock-Based Compensation - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017	Jan. 02, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 20,400,000	$ 25,400,000	$ 49,900,000
Unrecognized compensation cost		$ 26,300,000
Weighted average contractual term, unrecognized compensation cost		1 year 9 months 18 days
2015 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)					13,500,000
Shares available for grant (in shares)		5,600,000
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted during the period (in shares)		1,238,530
Intrinsic value, options outstanding		$ 37,100,000
Weighted average remaining contractual term		1 year
Restricted stock | 2015 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted during the period (in shares)		65,144	38,232	34,928
Award vesting period		1 year
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, granted (in shares)		0	0
Intrinsic value, units outstanding		$ 10,700,000
Weighted average remaining contractual term		5 years 6 months
Weighted average strike price, outstanding (in dollars per share)		$ 18.81
Exercisable (in shares)		2,000,000.0
Intrinsic value, exercisable stock options		$ 10,700,000
Weighted average strike price, exercisable (in dollars per share)		$ 18.54
Weighed average remaining contractual term, exercisable		5 years 6 months
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted during the period (in shares)		242,007
Award vesting period			3 years
Intrinsic value, units outstanding		$ 9,300,000
Weighted average remaining contractual term		1 year 6 months
Summit Materials, LLC
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 20,400,000	$ 25,400,000	$ 49,900,000
Unrecognized compensation cost		$ 26,300,000
Weighted average contractual term, unrecognized compensation cost		1 year 9 months 18 days
Weighed average remaining contractual term, exercisable		5 years 6 months
Summit Materials, LLC | Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted during the period (in shares)		1,238,530
Intrinsic value, options outstanding		$ 0
Intrinsic value, units outstanding		$ 37,100,000
Weighted average remaining contractual term		1 year
Summit Materials, LLC | Restricted stock | 2015 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		1 year
Summit Materials, LLC | Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, granted (in shares)		0	0
Intrinsic value, options outstanding		$ 10,700,000
Weighted average remaining contractual term		5 years 6 months
Weighted average strike price, outstanding (in dollars per share)		$ 18.81
Exercisable (in shares)		2,000,000.0
Intrinsic value, exercisable stock options		$ 10,700,000
Weighted average strike price, exercisable (in dollars per share)		$ 18.54
Summit Materials, LLC | Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted during the period (in shares)		242,007
Award vesting period		3 years
Intrinsic value, options outstanding		$ 0
Intrinsic value, units outstanding		$ 9,300,000
Weighted average remaining contractual term		1 year 6 months
Summit Holdings LP | Leverage restoration options, time-vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1,000,000.0
Minimum | Restricted stock | 2015 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			2 years
Minimum | Summit Materials, LLC | Restricted stock | 2015 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			2 years
Maximum | Restricted stock | 2015 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Maximum | Summit Materials, LLC | Restricted stock | 2015 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years