FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Book Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial assets:
Investment securities available-for-sale	$ 2,334,922	$ 1,944,914
Investment securities held-to-maturity, net	113,616	133,601
Net loans and leases	9,102,332	8,424,464
Stock in FHLB of Pittsburgh	6,497	21,097
Accrued interest receivable	42,801	38,094
Financial liabilities:
Deposits	11,391,345	9,586,857
Accrued interest payable	8,522	3,103
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash, cash equivalents, and restricted cash	1,072,920	571,752
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,334,922	1,944,914
Investment securities held-to-maturity, net	113,609	133,601
Loans held-for-sale	152,453	83,872
Stock in FHLB of Pittsburgh	6,497	21,097
Accrued interest receivable	42,801	38,094
Other assets	18,448	4,884
Financial liabilities:
Deposits	11,391,345	9,586,857
Borrowed funds	204,294	489,288
Accrued interest payable	8,522	3,103
Other liabilities	33,555	3,918
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	10,436	70,046
Net loans and leases	9,102,332	8,424,464
Financial liabilities:
Standby letters of credit	496	623
Other liabilities	33,555	3,918
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash, cash equivalents, and restricted cash	1,072,920	571,752
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,334,922	1,944,914
Investment securities held-to-maturity, net	118,113	136,625
Loans held-for-sale	152,453	83,872
Stock in FHLB of Pittsburgh	6,497	21,097
Accrued interest receivable	42,801	38,094
Other assets	18,448	4,884
Financial liabilities:
Deposits	11,353,731	9,575,394
Borrowed funds	206,804	489,561
Accrued interest payable	8,522	3,103
Other liabilities	33,555	3,918
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	10,436	70,046
Net loans and leases	9,455,051	8,580,015
Financial liabilities:
Standby letters of credit	496	623
Other liabilities	$ 33,555	$ 3,918