BORROWED FUNDS - Summary of Borrowed Funds (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Short-term borrowings	$ 616,312	$ 394,480
Long-term borrowings	268,916	142,116
Total borrowings	$ 885,228	$ 536,596
Weighted average rate on short-term borrowings	1.05%	0.43%
Weighted average rate on long-term borrowings	7.98%	1.69%
Weighted Average Rate	1.33%	1.13%
Advances from the FHLBB
Debt Instrument [Line Items]
Short-term borrowings	$ 582,755	$ 372,700
Long-term borrowings	$ 225,838	$ 137,116
Weighted average rate on short-term borrowings	1.08%	0.97%
Weighted average rate on long-term borrowings	1.19%	1.59%
Other Borrowings
Debt Instrument [Line Items]
Short-term borrowings	$ 33,557	$ 21,780
Weighted average rate on short-term borrowings	0.45%	0.29%
Subordinated borrowings
Debt Instrument [Line Items]
Long-term borrowings	$ 38,078	$ 0
Weighted average rate on long-term borrowings	6.36%	0.00%
Junior subordinated borrowings
Debt Instrument [Line Items]
Long-term borrowings	$ 5,000	$ 5,000
Weighted average rate on long-term borrowings	4.75%	4.41%