ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Commercial Credit Exposure (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	100.00%	100.00%
Acquired nonimpaired loans		$ 3,200,000	$ 3,933,340
Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans		3.00%
Risk Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans		0.00%
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans		97.00%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		$ 6,335,952
Acquired nonimpaired loans		2,200,000
Commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		52,765
Commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		134,505
Commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		15,515
Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		1,202
Commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		203,987
Commercial | Risk Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		17,482
Commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		6,114,483
Commercial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		2,235,428
Acquired nonimpaired loans			782,160
Commercial | Commercial and industrial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		12,287
Commercial | Commercial and industrial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		78,809
Commercial | Commercial and industrial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		9,852
Commercial | Commercial and industrial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		1,179
Commercial | Commercial and industrial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		102,127
Commercial | Commercial and industrial | Risk Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		1,564
Commercial | Commercial and industrial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		2,131,737
Commercial | Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		1,296,466
Acquired nonimpaired loans			105,154
Commercial | Owner-occupied commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		0
Commercial | Owner-occupied commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		32,679
Commercial | Owner-occupied commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		4,037
Commercial | Owner-occupied commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		23
Commercial | Owner-occupied commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		36,739
Commercial | Owner-occupied commercial | Risk Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		6,757
Commercial | Owner-occupied commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		1,252,970
Commercial | Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		2,222,976
Acquired nonimpaired loans			1,575,527
Commercial | Commercial mortgages | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		40,478
Commercial | Commercial mortgages | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		23,017
Commercial | Commercial mortgages | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		1,626
Commercial | Commercial mortgages | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		0
Commercial | Commercial mortgages | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		65,121
Commercial | Commercial mortgages | Risk Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		8,670
Commercial | Commercial mortgages | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		2,149,185
Commercial | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		581,082
Acquired nonimpaired loans			$ 225,245
Commercial | Construction | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		0
Commercial | Construction | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		0
Commercial | Construction | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		0
Commercial | Construction | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		0
Commercial | Construction | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		0
Commercial | Construction | Risk Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		491
Commercial | Construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		$ 580,591