DISCLOSURES ABOUT FAIR VALUES OF FINANCIAL INSTRUMENTS - Carrying amounts and estimated fair values (Details) - USD ($)
$ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
Mar. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
| Financial assets: |
|
|
|
|
|
| Cash and cash equivalents |
$ 23,726
|
$ 28,613
|
$ 22,320
|
$ 47,689
|
$ 10,705
|
| Available-for-sale securities |
49,662
|
51,403
|
|
61,429
|
|
| Federal Home Loan Bank of Boston stock |
3,515
|
2,650
|
|
1,854
|
|
| Accrued interest receivable |
3,171
|
2,638
|
|
2,345
|
|
| Financial liabilities: |
|
|
|
|
|
| Borrowings |
79,942
|
68,022
|
|
26,841
|
|
| Carrying Amount |
|
|
|
|
|
| Financial assets: |
|
|
|
|
|
| Cash and cash equivalents |
23,726
|
28,613
|
|
47,689
|
|
| Available-for-sale securities |
49,662
|
51,403
|
|
61,429
|
|
| Federal Home Loan Bank of Boston stock |
3,515
|
2,650
|
|
1,854
|
|
| Loans, net |
859,269
|
835,528
|
|
742,138
|
|
| Accrued interest receivable |
3,171
|
2,638
|
|
2,345
|
|
| Financial liabilities: |
|
|
|
|
|
| Deposits |
775,277
|
768,096
|
|
750,057
|
|
| Borrowings |
79,942
|
68,022
|
|
26,841
|
|
| Fair Value |
|
|
|
|
|
| Financial assets: |
|
|
|
|
|
| Cash and cash equivalents |
23,726
|
28,613
|
|
47,689
|
|
| Available-for-sale securities |
49,662
|
51,403
|
|
61,429
|
|
| Federal Home Loan Bank of Boston stock |
3,515
|
2,650
|
|
1,854
|
|
| Loans, net |
852,305
|
827,090
|
|
745,637
|
|
| Accrued interest receivable |
3,171
|
2,638
|
|
2,345
|
|
| Financial liabilities: |
|
|
|
|
|
| Deposits |
775,554
|
768,010
|
|
749,898
|
|
| Borrowings |
79,984
|
67,846
|
|
26,655
|
|
| Level 1 |
|
|
|
|
|
| Financial assets: |
|
|
|
|
|
| Cash and cash equivalents |
23,726
|
28,613
|
|
47,689
|
|
| Available-for-sale securities |
0
|
0
|
|
0
|
|
| Federal Home Loan Bank of Boston stock |
3,515
|
2,650
|
|
1,854
|
|
| Loans, net |
0
|
0
|
|
0
|
|
| Accrued interest receivable |
0
|
0
|
|
0
|
|
| Financial liabilities: |
|
|
|
|
|
| Deposits |
0
|
0
|
|
0
|
|
| Borrowings |
0
|
0
|
|
0
|
|
| Level 2 |
|
|
|
|
|
| Financial assets: |
|
|
|
|
|
| Cash and cash equivalents |
0
|
0
|
|
0
|
|
| Available-for-sale securities |
49,662
|
51,403
|
|
61,429
|
|
| Federal Home Loan Bank of Boston stock |
0
|
0
|
|
0
|
|
| Loans, net |
0
|
0
|
|
0
|
|
| Accrued interest receivable |
3,171
|
2,638
|
|
2,345
|
|
| Financial liabilities: |
|
|
|
|
|
| Deposits |
0
|
0
|
|
0
|
|
| Borrowings |
79,984
|
67,846
|
|
26,655
|
|
| Level 3 |
|
|
|
|
|
| Financial assets: |
|
|
|
|
|
| Cash and cash equivalents |
0
|
0
|
|
0
|
|
| Available-for-sale securities |
0
|
0
|
|
0
|
|
| Federal Home Loan Bank of Boston stock |
0
|
0
|
|
0
|
|
| Loans, net |
852,305
|
827,090
|
|
745,637
|
|
| Accrued interest receivable |
0
|
0
|
|
0
|
|
| Financial liabilities: |
|
|
|
|
|
| Deposits |
775,554
|
768,010
|
|
749,898
|
|
| Borrowings |
$ 0
|
$ 0
|
|
$ 0
|
|