Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Statutory federal tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal benefit	2.60%	2.10%	2.60%
Release of valuation allowance	(2.90%)
Audit settlements			(0.30%)
Federal research & development credits	(2.10%)	(0.60%)	(0.30%)
Other permanent differences	0.50%	0.10%	(0.50%)
Effective tax rate	19.10%	22.60%	22.50%