FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Book Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Investment securities available for sale	$ 1,523,196	$ 1,205,079
Investment securities held to maturity	148,190	149,950
Other investments	48,450	37,233
Loans, held for sale	33,893	25,318
Loans, net	8,655,604	4,863,919
Stock in FHLB of Pittsburgh	12,429	19,259
Accrued interest receivable	40,441	22,001
Other assets	296,354	65,020
Financial liabilities:
Deposits	9,673,704	5,640,431
Accrued interest payable	6,331	1,900
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	647,812	620,757
Book Value | Fair Value Inputs Level 1 And Level 2
Financial assets:
Investment securities available for sale	1,523,196	1,205,079
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	148,190	149,950
Loans, held for sale	33,893	25,318
Stock in FHLB of Pittsburgh	12,429	19,259
Accrued interest receivable	40,441	22,001
Financial liabilities:
Deposits	9,673,704	5,640,431
Borrowed funds	406,368	699,788
Accrued interest payable	6,331	1,900
Other liabilities	5,315	3,493
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	48,450	37,233
Loans, net	8,610,280	4,816,825
Impaired loans, net	45,324	47,094
Other assets	4,020	2,098
Financial liabilities:
Standby letters of credit	410	495
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	647,812	620,757
Fair Value | Fair Value Inputs Level 1 And Level 2
Financial assets:
Investment securities available for sale	1,523,196	1,205,079
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	149,732	149,431
Loans, held for sale	33,893	25,318
Stock in FHLB of Pittsburgh	12,429	19,259
Accrued interest receivable	40,441	22,001
Financial liabilities:
Deposits	9,737,150	5,597,227
Borrowed funds	219,430	694,526
Accrued interest payable	6,331	1,900
Other liabilities	5,315	3,493
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	48,450	37,233
Loans, net	8,672,826	4,772,377
Impaired loans, net	45,324	47,094
Other assets	4,020	2,098
Financial liabilities:
Standby letters of credit	$ 410	$ 495