LOANS - Purchased Credit Impaired Loans (Details) - USD ($) $ in Thousands	1 Months Ended		9 Months Ended
	Jun. 19, 2015	Feb. 14, 2014	Sep. 30, 2018	Dec. 31, 2017
Financing Receivables
Total loans			$ 3,191,408	$ 3,098,253
Impaired loans			15,600	22,500
Nonaccrual troubled debt restructured loans			146	5
Amount of current and performing TDR loans			16,700	16,700
Post-Modification of other than troubled debt restructuring, recorded investment			41,100
Impaired Loans			15,550	22,492
Other real estate owned			$ 175
Period of modified contractually past due loans to be considered as payment default			30 days
Other real estate owned
Financing Receivables
Impaired Loans			$ 200	0
Special Mention
Financing Receivables
Total loans			25,283	42,610
Substandard
Financing Receivables
Total loans			46,210	42,683
FNBNY
Acquired Loans
Contractually required principal and interest payments receivable		$ 40,300
Expected cash flows		28,400
Fair value (initial carrying amount) of purchased credit impaired loans		21,800
Non-accretable difference		11,900
Initial accretable yield		$ 6,600
Outstanding balance of purchased credit impaired loans			1,200	4,000
Carrying amount of purchased credit impaired loans			500	2,400
Remaining non-accretable difference			500	700
FNBNY | Special Mention
Financing Receivables
Total loans			200	200
FNBNY | Substandard
Financing Receivables
Total loans			300	300
CNB
Acquired Loans
Contractually required principal and interest payments receivable	$ 23,400
Expected cash flows	10,100
Fair value (initial carrying amount) of purchased credit impaired loans	8,700
Non-accretable difference	13,300
Initial accretable yield	$ 1,400
Outstanding balance of purchased credit impaired loans			1,300	7,600
Carrying amount of purchased credit impaired loans			100	1,000
Remaining non-accretable difference			800	5,300
CNB | 30-89 days past due
Acquired Loans
Acquired loans			1,100	2,400
CNB | Special Mention
Financing Receivables
Total loans			400	400
CNB | Substandard
Financing Receivables
Total loans			$ 800	$ 1,600