SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Impact of ASC 326 (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 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	139,073		47,576	$ 46,321
Retained earnings	891,776		917,377
Commercial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	65,771		22,849	21,016
Commercial | Owner-occupied commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	9,541		4,616	4,949
Commercial | Commercial mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	26,600		7,452	6,679
Commercial | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	5,198		3,891	4,044
Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	11,593		1,381	1,401
Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	20,370		7,387	$ 8,232
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)