Stockholders' Equity - Common Stock Warrants (Details) - Common Stock - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	May 07, 2018
Common Stock Warrants
Aggregate number of shares called by warrants (in shares)		76,762	100,558
Exercised (in shares)	0	93,963
Warrants, weighted average exercise price (in dollars per share)			$ 9.38
Warrants
Outstanding at beginning of the year (in shares)	6,595
Exercised (in shares)	0	(93,963)
Outstanding at end of the year (in shares)	6,595	6,595
Weighted Average Exercise Price]
Outstanding, beginning of the period (in dollars per share)	$ 15.16
Exercised (in dollars per share)	0
Outstanding, end of the period (in dollars per share)	$ 15.16	$ 15.16
Weighted average remaining contractual term
Outstanding (in years)	5 years 3 months 18 days	5 years 6 months