SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Impact of ASC 326 (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities	$ 8			$ 136,625
Allowance for credit losses	232,192	$ 139,073		47,576	$ 45,364
Retained earnings	878,585			917,377
Commercial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	144,225	65,771		22,849	22,004
Commercial | Owner-occupied commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	8,956	9,541		4,616	4,480
Commercial | Commercial mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	38,397	26,600		7,452	6,544
Commercial | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	10,126	5,198		3,891	2,984
Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	9,171	11,593		1,381	1,358
Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	21,317	$ 20,370		7,387	$ 7,994
State and political subdivisions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities	$ 8			0
Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				35,855
Impact of ASC 326 Adoption | Commercial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				19,747
Impact of ASC 326 Adoption | Commercial | Owner-occupied commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				(1,472)
Impact of ASC 326 Adoption | Commercial | Commercial mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				1,662
Impact of ASC 326 Adoption | Commercial | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				681
Impact of ASC 326 Adoption | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				7,522
Impact of ASC 326 Adoption | Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				7,715
Impact of ASC 326 Adoption | State and political subdivisions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities				$ 8
ASC 326
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities			$ 0
Allowance for credit losses			(47,576)
Deferred tax assets			9,991
Allowance for credit losses on unfunded lending commitments			(1,547)
ASC 326 | Commercial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(22,849)
ASC 326 | Commercial | Owner-occupied commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(4,616)
ASC 326 | Commercial | Commercial mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(7,452)
ASC 326 | Commercial | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(3,891)
ASC 326 | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(1,381)
ASC 326 | Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(7,387)
ASC 326 | State and political subdivisions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities			0
ASC 326 | As reported under ASC 326
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities			(8)
Allowance for credit losses			(83,431)
Deferred tax assets			18,452
Allowance for credit losses on unfunded lending commitments			(4,513)
Retained earnings			30,400
ASC 326 | As reported under ASC 326 | Commercial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(42,596)
ASC 326 | As reported under ASC 326 | Commercial | Owner-occupied commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(3,144)
ASC 326 | As reported under ASC 326 | Commercial | Commercial mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(9,114)
ASC 326 | As reported under ASC 326 | Commercial | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(4,572)
ASC 326 | As reported under ASC 326 | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(8,903)
ASC 326 | As reported under ASC 326 | Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(15,102)
ASC 326 | As reported under ASC 326 | State and political subdivisions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities			(8)
ASC 326 | Impact of ASC 326 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities			(8)
Allowance for credit losses			(35,855)
Deferred tax assets			8,461
Allowance for credit losses on unfunded lending commitments			(2,966)
Retained earnings			30,368
ASC 326 | Impact of ASC 326 Adoption | Commercial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(19,747)
ASC 326 | Impact of ASC 326 Adoption | Commercial | Owner-occupied commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			1,472
ASC 326 | Impact of ASC 326 Adoption | Commercial | Commercial mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(1,662)
ASC 326 | Impact of ASC 326 Adoption | Commercial | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(681)
ASC 326 | Impact of ASC 326 Adoption | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(7,522)
ASC 326 | Impact of ASC 326 Adoption | Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			(7,715)
ASC 326 | Impact of ASC 326 Adoption | State and political subdivisions
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on held-to-maturity debt securities			$ (8)