Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19-26)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_Liabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Commitments and Contingencies - Pfizer License Agreement (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Commitments and Contingencies
No liabilities were recorded related to the license agreement	$ 2,050,000	$ 11,920,000	$ 15,076,000
Pfizer, Inc.
Commitments and Contingencies
No liabilities were recorded related to the license agreement	$ 0