Income Taxes - Summary of reconciliation between federal statutory rate and effective tax rate (Details)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Federal statutory rate	21.00%	21.00%	21.00%	21.00%	35.00%
State income taxes, net of federal benefit		1.50%	(0.30%)	2.60%
Tax credits		0.10%	0.30%	(0.10%)
Non-deductible expenses		(0.10%)	(1.90%)	1.80%
Adjustments related to CARES Act		8.50%
Adjustments related to the Act				(2.30%)
Adjustments related to vesting of restricted stock		(0.80%)	(0.40%)	(0.80%)
Other		1.20%	4.30%	(1.50%)
Effective tax rate		31.40%	23.00%	20.70%