GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Identifiable Intangible Assets [Abstract]
Gross carrying amount	$ 155,741	$ 150,518
Accumulated amortization	(92,628)	(76,672)
Net carrying amount	63,113	73,846
Estimated aggregate amortization expense for each of five succeeding fiscal years [Abstract]
2020	13,800
2021	11,786
2022	10,105
2023	8,457
2024	7,041
Thereafter	11,924
Net carrying amount	63,113	73,846
Components of goodwill [Abstract]
Goodwill, beginning of period	738,327	739,254
Goodwill, activity	40,307
Goodwill, activity		(927)
Goodwill, end of period	778,634	738,327
Accumulated impairment, beginning of period	(4,824)	(4,824)
Accumulated impairment, activity	0	0
Accumulated impairment, end of period	(4,824)	(4,824)
Goodwill, net, beginning of period	733,503	734,430
Goodwill, net, activity	40,307	(927)
Goodwill, net, end of period	773,810	733,503
Servicing Asset at Fair Value, Amount [Roll Forward]
Carrying value before valuation allowance at beginning of period	1,137	1,358
Additions	17	228
Kinderhook acquisition	196	0
Amortization	(378)	(449)
Carrying value before valuation allowance at end of period	972	1,137
Valuation allowance balance at beginning of period	0	0
Impairment charges	(326)	0
Impairment recoveries	28	0
Valuation allowance balance at end of period	(298)	0
Net carrying value at end of period	674	1,137
Fair value of MSRs at end of period	1,362	2,397
Principal balance of loans sold during the year	2,204	18,228
Principal balance of loans serviced for others	294,093	333,862
Custodial escrow balances maintained in connection with loans serviced for others	$ 4,596	$ 4,982
Key economic assumptions used to estimate the value of the MSRs [Abstract]
Weighted-average contractual life	20 years 10 months 24 days	21 years 4 months 24 days
Weighted-average constant prepayment rate (CPR)	18.70%	9.30%
Weighted-average discount rate	3.00%	3.60%
Core Deposit Intangibles [Member]
Identifiable Intangible Assets [Abstract]
Gross carrying amount	$ 66,475	$ 62,902
Accumulated amortization	(50,057)	(44,306)
Net carrying amount	16,418	18,596
Estimated aggregate amortization expense for each of five succeeding fiscal years [Abstract]
Net carrying amount	16,418	18,596
Other Intangibles [Member]
Identifiable Intangible Assets [Abstract]
Gross carrying amount	89,266	87,616
Accumulated amortization	(42,571)	(32,366)
Net carrying amount	46,695	55,250
Estimated aggregate amortization expense for each of five succeeding fiscal years [Abstract]
Net carrying amount	$ 46,695	$ 55,250