FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Book Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financial assets:
Investment securities available-for-sale	$ 2,195,389	$ 1,944,914
Investment securities held-to-maturity, net	127,609	133,601
Net loans and leases	9,120,288	8,424,464
Stock in FHLB of Pittsburgh	9,772	21,097
Accrued interest receivable	53,222	38,094
Financial liabilities:
Deposits	11,062,500	9,586,857
Accrued interest payable	6,049	3,103
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	955,767	571,752
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,195,389	1,944,914
Investment securities held-to-maturity, net	127,601	133,601
Loans held-for-sale	109,453	83,872
Stock in FHLB of Pittsburgh	9,772	21,097
Accrued interest receivable	53,222	38,094
Other assets	15,243	4,884
Financial liabilities:
Deposits	11,062,500	9,586,857
Borrowed funds	295,793	489,288
Accrued interest payable	6,049	3,103
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	10,211	70,046
Net loans and leases	9,120,288	8,424,464
Financial liabilities:
Standby letters of credit	416	623
Book Value | Levels 2, 3
Financial liabilities:
Other liabilities	34,222	3,918
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	955,767	571,752
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,195,389	1,944,914
Investment securities held-to-maturity, net	132,198	136,625
Loans held-for-sale	109,453	83,872
Stock in FHLB of Pittsburgh	9,772	21,097
Accrued interest receivable	53,222	38,094
Other assets	15,243	4,884
Financial liabilities:
Deposits	11,052,211	9,575,394
Borrowed funds	313,839	489,561
Accrued interest payable	6,049	3,103
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	10,211	70,046
Net loans and leases	9,386,635	8,580,015
Financial liabilities:
Standby letters of credit	416	623
Fair Value | Levels 2, 3
Financial liabilities:
Other liabilities	$ 34,222	$ 3,918