Income Taxes (Tax Rate Reconciliation) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Federal statutory rate	21.00%	21.00%	35.00%
State taxes	1.50%	(1.90%)	(2.10%)
Stock-based compensation	(7.20%)	(20.40%)	(58.10%)
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent	0.10%	(0.20%)	(0.20%)
Change in valuation allowance	1.10%	4.40%	2.80%
Incremental federal rate benefit previously not recognized	0.00%	0.00%	26.40%
Federal and state research and development credit	(3.70%)	(6.70%)	(5.30%)
Other	0.40%	0.50%	(1.20%)
Provision for (benefit from) income taxes	13.20%	(3.30%)	(2.70%)