INCOME TAXES - Gross deferred tax assets and gross deferred tax liabilities (Details 2) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Deferred tax assets:      
Allowance for loan losses   $ 3,251 $ 2,743
Depreciation   160 41
Net operating loss carryforward   16 25
Employee benefit plans and share-based compensation plans   2,498 1,979
Deferred loan fees, net   339 238
Reserve for unfunded commitments   31 39
Net unrealized loss on securities   107 0
Other   109 140
Gross deferred tax assets   6,511 5,205
Deferred tax liabilities:      
Prepaid expenses   (45) (64)
FHLB restructure fees   (29) (52)
Net unrealized holding gain on securities   0 (169)
Gross deferred tax liabilities   (74) (285)
Net deferred tax asset $ 6,589 $ 6,437 $ 4,920