REGULATORY MATTERS - Narrative (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common equity tier 1 capital to risk-weighted assets:
Capital conservation buffer percentage, increase		0.625%	0.625%	0.625%
Subordinated debentures | Subordinated Debentures II, Subordinated Debentures III, and DCB Debentures I
Common equity tier 1 capital to risk-weighted assets:
Debenture issued	$ 10,310,000	$ 10,310,000
Minimum
Common equity tier 1 capital to risk-weighted assets:
Capital conservation buffer percentage					0.625%
Maximum
Common equity tier 1 capital to risk-weighted assets:
Capital conservation buffer percentage		2.50%