SUMMARY OF SIGNIFICANT ACCOUNTING AND REPORTING POLICIES - Reclassifications (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization expense on intangibles	$ 5,213,000	$ 1,955,000	$ 1,940,000
Securities available for sale, at estimated fair value		1,538,755,000
Securities held to maturity, at carrying value (estimated fair value of $159,781 and $921,800, respectively)	162,931,000	909,506,000
Decrease in debit card expense	1,090,000	3,889,000	3,136,000
Decrease in deposit services income	25,082,000	21,785,000	20,702,000
Decrease in brokerage services income	1,987,000	2,422,000	2,127,000
Cumulative effect of ASU 2016-01	(85,000)
Increase (decrease) in salaries and employee benefits expense	70,643,000	60,779,000	61,628,000
(Decrease) increase in other noninterest expense	12,283,000	10,528,000	14,524,000
Securities available for sale	2,013,485,000
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in debit card expense	4,000,000
Decrease in deposit services income	4,030,000
Decrease in brokerage expense	641,000
Decrease in brokerage services income	641,000
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities available for sale, at estimated fair value				$ (5,900,000)
Equity investments				5,900,000
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in salaries and employee benefits expense		304,000	(2,400,000)
(Decrease) increase in other noninterest expense		(304,000)	$ 2,400,000
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities held to maturity, at carrying value (estimated fair value of $159,781 and $921,800, respectively)				(743,400,000)
Securities available for sale				743,400,000
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of ASU 2016-01	(85,000)
Retained Earnings | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of ASU 2016-01				$ (85,000)
Core Deposit Intangible and Trust Relationship Intangible [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization expense on intangibles	$ 5,100,000	$ 1,800,000