FAIR VALUE MEASUREMENTS - Fair Values and Carrying Amounts (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Financial Assets
Securities available for sale	$ 718,346	$ 528,856
Level 1
Financial Assets
Cash and cash equivalents	118,993	8,439
Securities available for sale	0	0
FHLBB bank stock	0	0
Net loans	0	0
Accrued interest receivable	0	0
Cash surrender value of bank-owned life insurance policies	0	0
Derivative assets	0	0
Financial Liabilities
Total deposits	0	0
Securities sold under agreements to repurchase	0	0
Federal Home Loan Bank advances	0	0
Derivative liabilities	0
Level 1 | Senior subordinated
Financial Liabilities
Subordinated borrowings	0	0
Level 1 | Junior subordinated
Financial Liabilities
Subordinated borrowings	0	0
Level 2
Financial Assets
Cash and cash equivalents	0	0
Securities available for sale	718,364	528,856
FHLBB bank stock	44,168	25,331
Net loans	0	0
Accrued interest receivable	3,386	6,051
Cash surrender value of bank-owned life insurance policies	57,233	24,450
Derivative assets	950	1,748
Financial Liabilities
Total deposits	2,190,676	1,048,932
Securities sold under agreements to repurchase	35,155	21,773
Federal Home Loan Bank advances	837,126	509,793
Derivative liabilities	0
Level 2 | Senior subordinated
Financial Liabilities
Subordinated borrowings	37,921	0
Level 2 | Junior subordinated
Financial Liabilities
Subordinated borrowings	3,569	3,560
Level 3
Financial Assets
Cash and cash equivalents	0	0
Securities available for sale	0	0
FHLBB bank stock	0	0
Net loans	2,342,877	1,100,601
Accrued interest receivable	0	0
Cash surrender value of bank-owned life insurance policies	0	0
Derivative assets		0
Financial Liabilities
Total deposits	0	0
Securities sold under agreements to repurchase	0	0
Federal Home Loan Bank advances	0	0
Derivative liabilities	(234)
Level 3 | Senior subordinated
Financial Liabilities
Subordinated borrowings	0	0
Level 3 | Junior subordinated
Financial Liabilities
Subordinated borrowings	0	0
Carrying Amount
Financial Assets
Cash and cash equivalents	118,993	8,439
Securities available for sale	718,364	528,856
FHLBB bank stock	44,168	25,331
Net loans	2,365,205	1,118,645
Accrued interest receivable	3,386	6,051
Cash surrender value of bank-owned life insurance policies	57,233	24,450
Derivative assets	950	1,748
Financial Liabilities
Total deposits	2,213,004	1,050,300
Securities sold under agreements to repurchase	35,174	21,780
Federal Home Loan Bank advances	837,267	509,816
Derivative liabilities	(234)
Carrying Amount | Senior subordinated
Financial Liabilities
Subordinated borrowings	37,642	0
Carrying Amount | Junior subordinated
Financial Liabilities
Subordinated borrowings	5,000	0
Total Fair Value
Financial Assets
Cash and cash equivalents	118,993	8,439
Securities available for sale	718,364	528,856
FHLBB bank stock	44,168	25,331
Net loans	2,342,877	1,100,601
Accrued interest receivable	3,386	6,051
Cash surrender value of bank-owned life insurance policies	57,233	24,450
Derivative assets	950	1,748
Financial Liabilities
Total deposits	2,190,676	1,048,932
Securities sold under agreements to repurchase	35,155	21,773
Federal Home Loan Bank advances	837,126	509,793
Derivative liabilities	(234)
Total Fair Value | Senior subordinated
Financial Liabilities
Subordinated borrowings	37,921	0
Total Fair Value | Junior subordinated
Financial Liabilities
Subordinated borrowings	$ 3,569	$ 3,560