FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Book Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial assets:
Investment securities available for sale	$ 1,908,821	$ 1,205,079
Investment securities held to maturity	134,961	149,950
Loans, net	8,480,712	4,863,919
Stock in FHLB of Pittsburgh	22,630	19,259
Accrued interest receivable	38,771	22,001
Financial liabilities:
Deposits	9,533,193	5,640,431
Accrued interest payable	9,481	1,900
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	580,299	620,757
Book Value | Fair Value Inputs Level 1 And Level 2
Financial assets:
Investment securities available for sale	1,908,821	1,205,079
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	134,961	149,950
Loans held-for-sale	67,428	25,318
Stock in FHLB of Pittsburgh	22,630	19,259
Accrued interest receivable	38,771	22,001
Financial liabilities:
Deposits	9,533,193	5,640,431
Borrowed funds	554,783	699,788
Accrued interest payable	9,481	1,900
Other liabilities	5,974	3,493
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	70,055	37,233
Loans, net	8,430,176	4,816,825
Impaired loans, net	50,536	47,094
Other assets	6,540	2,098
Financial liabilities:
Standby letters of credit	461	495
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	580,299	620,757
Fair Value | Fair Value Inputs Level 1 And Level 2
Financial assets:
Investment securities available for sale	1,908,821	1,205,079
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	138,262	149,431
Loans held-for-sale	67,428	25,318
Stock in FHLB of Pittsburgh	22,630	19,259
Accrued interest receivable	38,771	22,001
Financial liabilities:
Deposits	9,835,611	5,597,227
Borrowed funds	555,555	694,526
Accrued interest payable	9,481	1,900
Other liabilities	5,974	3,493
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	70,055	37,233
Loans, net	8,530,619	4,772,377
Impaired loans, net	50,536	47,094
Other assets	6,540	2,098
Financial liabilities:
Standby letters of credit	$ 461	$ 495