Stock Options - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands						3 Months Ended
	Mar. 15, 2019	Aug. 24, 2018	Jan. 05, 2018	Mar. 24, 2017	Aug. 27, 2015	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Employee Stock Purchase Plan
Options, outstanding (in shares)					23,500	423,700		118,500	263,700	93,500
Granted (in shares)					38,750	160,000		25,000
Granted (in dollars per share)					$ 42.03	$ 42.01		$ 45.11
Expiration (in years)					10 years
Exercisable period (in years)					5 years
Employee Stock Option
Employee Stock Purchase Plan
Expiration (in years)						10 years
Compensation expense						$ 136		$ 7
Number of exercisable options (in shares)						0
Weighted average remaining contractual term (in years)						9 years 2 months 8 days
Fair value of options granted						$ 1,208
Fair value of options granted (in dollars per share)							$ 7.55
Unrecognized compensation cost for non-vested shares						$ 2,528
Period for recognition (in years)						3 years 8 months 9 days
Tranche One
Employee Stock Purchase Plan
Options, outstanding (in shares)	80,600	50,200	12,500	41,750
Granted (in shares)	80,600	50,200	12,500	46,250
Granted (in dollars per share)	$ 42.01	$ 46.00	$ 45.11	$ 44.21
Expiration (in years)	10 years	10 years	10 years	10 years
Exercisable period (in years)	3 years	3 years	3 years	3 years
Tranche One | Employee Stock Option
Employee Stock Purchase Plan
Exercisable period (in years)	3 years
Tranche Two
Employee Stock Purchase Plan
Options, outstanding (in shares)	79,400	99,500	12,500	23,750
Granted (in shares)	79,400	99,500	12,500	23,750
Granted (in dollars per share)	$ 42.01	$ 46.00	$ 45.11	$ 44.21
Expiration (in years)	10 years	10 years	10 years	10 years
Exercisable period (in years)	5 years	5 years	5 years	5 years
Tranche Two | Employee Stock Option
Employee Stock Purchase Plan
Exercisable period (in years)	5 years