ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands				3 Months Ended
		Sep. 18, 2019	Jul. 12, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Consideration paid [Abstract]
Cash		$ 500	$ 95,034	$ (0)	$ 1,200
Total net consideration paid			95,034
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents			90,381
Investment securities			39,770
Loans			479,877
Premises and equipment			13,970
Accrued interest and fees receivable			1,130
Other assets			14,109
Deposits			(568,161)
Other liabilities			(2,922)
Other Federal Home Loan Bank borrowings			(2,420)
Subordinated notes payable			(13,831)
Subordinated debt held by unconsolidated subsidiary trusts			(2,062)
Total identifiable assets, net			55,064
Goodwill			39,970	$ 773,473	$ 733,503	$ 773,810
Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			3,573
Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			1,650
Kinderhook Bank Corp [Member]
Consideration paid [Abstract]
Cash			93,384
Total net consideration paid			93,384
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents			90,381
Investment securities			39,770
Loans			479,877
Premises and equipment			13,970
Accrued interest and fees receivable			1,130
Other assets			14,109
Deposits			(568,161)
Other liabilities			(2,922)
Other Federal Home Loan Bank borrowings			(2,420)
Subordinated notes payable			(13,831)
Subordinated debt held by unconsolidated subsidiary trusts			(2,062)
Total identifiable assets, net			53,414
Goodwill			39,970
Kinderhook Bank Corp [Member] | Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			3,573
Kinderhook Bank Corp [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles			0
Other [Member]
Consideration paid [Abstract]
Cash	[1]		1,650
Total net consideration paid	[1]		1,650
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Cash and cash equivalents	[1]		0
Investment securities	[1]		0
Loans	[1]		0
Premises and equipment	[1]		0
Accrued interest and fees receivable	[1]		0
Other assets	[1]		0
Deposits	[1]		0
Other liabilities	[1]		0
Other Federal Home Loan Bank borrowings	[1]		0
Subordinated notes payable	[1]		0
Subordinated debt held by unconsolidated subsidiary trusts	[1]		0
Total identifiable assets, net	[1]		1,650
Goodwill	[1]		0
Other [Member] | Core Deposit Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	[1]		0
Other [Member] | Other Intangibles [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed [Abstract]
Intangibles	[1]		$ 1,650

[1] Includes amounts related to both acquisitions completed by CISI in 2019.