FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Book Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financial assets:
Investment securities available-for-sale	$ 2,048,400	$ 1,944,914
Investment securities held-to-maturity, net	134,055	133,601
Loans, net	8,411,689	8,424,464
Stock in FHLB of Pittsburgh	33,988	21,097
Accrued interest receivable	39,134	38,094
Financial liabilities:
Deposits	9,709,354	9,586,857
Accrued interest payable	5,797	3,103
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	505,110	571,752
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,048,400	1,944,914
Investment securities held-to-maturity, net	134,047	133,601
Loans held-for-sale	85,458	83,872
Stock in FHLB of Pittsburgh	33,988	21,097
Accrued interest receivable	39,134	38,094
Other assets	12,364	4,884
Financial liabilities:
Deposits	9,709,354	9,586,857
Borrowed funds	401,285	489,288
Accrued interest payable	5,797	3,103
Other liabilities	9,325	3,918
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	70,714	70,046
Loans, net	8,411,689	8,424,464
Financial liabilities:
Standby letters of credit	505	623
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	505,110	571,752
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available-for-sale	2,048,400	1,944,914
Investment securities held-to-maturity, net	137,374	136,625
Loans held-for-sale	85,458	83,872
Stock in FHLB of Pittsburgh	33,988	21,097
Accrued interest receivable	39,134	38,094
Other assets	12,364	4,884
Financial liabilities:
Deposits	9,710,432	9,575,394
Borrowed funds	404,955	489,561
Accrued interest payable	5,797	3,103
Other liabilities	9,325	3,918
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	70,714	70,046
Loans, net	8,475,886	8,580,015
Financial liabilities:
Standby letters of credit	$ 505	$ 623