Income Taxes - Reconciliation (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Jan. 01, 2018	Sep. 30, 2019	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes
Effective tax rates on continuing operations				24.70%	24.10%	45.20%
Federal statutory income tax rate (as a percent)	21.00%			21.00%	21.00%	35.00%
Income taxes at the federal statutory rate of 35%				$ 31,866	$ 31,222	$ 35,328
Increases (decreases) resulting from-
Net state income taxes				6,644	7,470	2,838
Valuation allowances				(279)	(2,852)	91
Net unrecognized tax benefits				7,338	(15)	153
Nondeductible expenses				2,180	1,926	1,134
R&D tax credits		$ (4,600)		(4,569)	(2,726)
179D deduction				(5,126)
Net operating loss carryforwards					2,225
Stock-based compensation deductions				(714)	(1,293)	(1,320)
Domestic production activities deduction						(2,112)
Corporate tax rate reduction to 21%						9,478
Other				78	(184)	76
Provision for income taxes				37,418	$ 35,773	$ 45,666
Increase to provision for income taxes			$ 2,800
179D deduction				$ (2,200)