Stock-Based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018	Jan. 02, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 28.9	$ 20.4	$ 25.4
Dividend yield		0.00%
Unrecognized compensation cost		$ 22.0
Weighted average contractual term, unrecognized compensation cost		1 year 8 months 12 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)		4,300,000
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted during the period (in shares)		1,379,943
Intrinsic value, units outstanding		$ 42.9
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]
Award vesting period		1 year
Awards granted during the period (in shares)		42,736	65,144	38,232
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, granted (in shares)		0	0	0
Intrinsic value, options outstanding		$ 3.8
Weighted average remaining contractual term		3 years 3 months 18 days
Weighted average strike price, outstanding (in dollars per share)		$ 18.75
Exercisable (in shares)		2,000,000.0
Intrinsic value, exercisable stock options		$ 3.8
Weighted average strike price, exercisable (in dollars per share)		$ 18.75
Weighed average remaining contractual term, exercisable		3 years 3 months 18 days
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		3 years
Awards granted during the period (in shares)		199,946
Intrinsic value, units outstanding		$ 8.6
Weighted average remaining contractual term		1 year 3 months 18 days
Summit Materials, LLC
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 28.9	$ 20.4	$ 25.4
Dividend yield		0.00%
Unrecognized compensation cost		$ 22.0
Weighted average contractual term, unrecognized compensation cost			1 year 8 months 12 days
Weighed average remaining contractual term, exercisable		3 years 3 months 18 days
Summit Materials, LLC | Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards granted during the period (in shares)		1,379,943
Intrinsic value, units outstanding		$ 42.9
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 | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, granted (in shares)		0	0	0
Intrinsic value, options outstanding		$ 3.8
Weighted average remaining contractual term		3 years 3 months 18 days
Weighted average strike price, outstanding (in dollars per share)		$ 18.75
Exercisable (in shares)		2,000,000.0
Intrinsic value, exercisable stock options		$ 3.8
Weighted average strike price, exercisable (in dollars per share)		$ 18.75
Summit Materials, LLC | Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		3 years
Awards granted during the period (in shares)		199,946
Intrinsic value, units outstanding		$ 8.6
Weighted average remaining contractual term		1 year 3 months 18 days
Summit Holdings LP | Leverage restoration options, time-vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1.0
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