Income Taxes - Tax Credit Carryforwards (Details) - Research and development tax credit carryforward $ in Millions	Dec. 31, 2017 USD ($)
Tax credit carryforwards
Tax credit carryforwards	$ 13.5
Federal
Tax credit carryforwards
Tax credit carryforwards	3.5
State
Tax credit carryforwards
Tax credit carryforwards	$ 3.2

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

Name:	us-gaap_TaxCreditCarryforwardAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant