FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Book Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financial assets:
Investment securities available-for-sale	$ 2,987,885	$ 2,529,057
Investment securities held-to-maturity, net	103,528	111,747
Net loans and leases	8,373,766	8,795,935
Stock in FHLB of Pittsburgh	5,506	5,771
Accrued interest receivable	43,159	44,335
Financial liabilities:
Deposits	12,283,700	11,856,664
Accrued interest payable	4,153	1,450
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash, cash equivalents, and restricted cash	2,064,161	1,654,735
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,987,885	2,529,057
Investment securities held-to-maturity, net	103,523	111,741
Loans held-for-sale	155,447	197,541
Stock in FHLB of Pittsburgh	5,506	5,771
Accrued interest receivable	43,159	44,335
Other assets	11,439	14,448
Financial liabilities:
Deposits	12,283,700	11,856,664
Borrowed funds	335,201	340,641
Accrued interest payable	4,153	1,450
Other liabilities	26,783	32,257
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	10,227	9,541
Net loans and leases	8,373,766	8,795,935
Financial liabilities:
Standby letters of credit	552	630
Other liabilities	26,783	32,257
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash, cash equivalents, and restricted cash	2,064,161	1,654,735
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,987,885	2,529,057
Investment securities held-to-maturity, net	107,629	116,421
Loans held-for-sale	155,447	197,541
Stock in FHLB of Pittsburgh	5,506	5,771
Accrued interest receivable	43,159	44,335
Other assets	11,439	14,448
Financial liabilities:
Deposits	12,292,473	11,868,897
Borrowed funds	323,927	340,106
Accrued interest payable	4,153	1,450
Other liabilities	26,783	32,257
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	10,227	9,541
Net loans and leases	8,550,651	9,130,003
Financial liabilities:
Standby letters of credit	552	630
Other liabilities	$ 26,783	$ 32,257