Common and Preferred Stock - 2004, 2016 and 2018 Plans (Details) - shares	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Jul. 31, 2018
2004 Plan
Common and Preferred Stock
Shares reserved for issuance	399,069
2016 Plan
Common and Preferred Stock
Shares reserved for issuance	747,156
2018 Plan
Common and Preferred Stock
Shares reserved for issuance	2,217,735	2,220,778	1,600,000	1,600,000
ESPP
Common and Preferred Stock
Shares reserved for issuance	300,000