Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
Effective tax rates on continuing operations			24.10%	45.20%	35.80%
Federal statutory income tax rate (as a percent)	21.00%		21.00%	35.00%	35.00%
Income taxes at the federal statutory rate of 35%			$ 31,222	$ 35,328	$ 35,371
Increases (decreases) resulting from—
Net state income taxes			7,470	2,838	4,262
Valuation allowances			(2,852)	91	(1,254)
Net unrecognized tax benefits			(15)	153	20
Nondeductible expenses			1,926	1,134	825
R&D tax credits			(2,726)
Net operating loss carryforwards			2,225
Stock-based compensation deductions			(1,293)	(1,320)	(885)
Domestic production activities deduction				(2,112)	(2,026)
Corporate tax rate reduction to 21%				9,478
Other			(184)	76	(148)
Provision for income taxes			$ 35,773	45,666	$ 36,165
Increase to provision for income taxes		$ 9,500		$ 9,500