Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statutory federal income tax rate
Statutory federal income tax rate (as a percent)	21.00%	21.00%	35.00%
Deferred tax assets:
Valuation allowance against deferred tax assets	$ 115,307,000	$ 93,904,000
Net operating loss carryforwards from year of generation	20 years
Net operating loss carryforwards indefinitely subject to limitation	80.00%
Tax Credit Carryforwards
Unrecognized tax benefits	$ 1,116,000	1,076,000	$ 966,000
Unrecognized Tax Benefits Penalties and Interest
Interest expense or penalties related to unrecognized tax benefits	0
Research and development
Tax Credit Carryforwards
Tax credit carryforwards	2,100,000	$ 2,700,000
Unrecognized tax benefits	1,100,000
Federal
Deferred tax assets:
Net operating loss carryforwards	$ 388,900,000
Federal | Minimum
Income Tax Uncertainties [Abstract]
Tax years	2016
Federal | Maximum
Income Tax Uncertainties [Abstract]
Tax years	2018
Federal | Carried forward for 20 years
Deferred tax assets:
Net operating loss carryforwards			$ 235,300,000
Federal | Carried forward indefinitely
Deferred tax assets:
Net operating loss carryforwards	$ 153,500,000
State
Deferred tax assets:
Net operating loss carryforwards	$ 224,900,000
Net operating loss carryforwards, expiration year	2017
State | Minimum
Income Tax Uncertainties [Abstract]
Tax years	2015
State | Maximum
Income Tax Uncertainties [Abstract]
Tax years	2018