FAIR VALUE DISCLOSURES OF FINANCIAL ASSETS AND LIABILITIES - Book Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Investment securities available for sale		$ 838,869
Investment securities held to maturity	$ 152,577	161,186
Other investments	35,083	17,971
Loans, held for sale	35,855	31,055
Loans, net	4,886,136	4,776,318
Stock in FHLB of Pittsburgh	16,540	31,284
Accrued interest receivable	21,331	19,405
Other assets	62,041	59,068
Financial liabilities:
Deposits	5,723,916	5,247,604
Accrued interest payable	7,119	1,037
Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Investment securities available for sale		623
Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities available for sale		838,246
Significant Unobservable Inputs (Level 3)
Financial assets:
Investment securities available for sale		0
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	711,372	723,866
Book Value | Fair Value Inputs Level 1 And Level 2
Financial assets:
Investment securities available for sale	997,131	837,499
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	152,577	161,186
Loans, held for sale	35,855	31,055
Stock in FHLB of Pittsburgh	16,540	31,284
Accrued interest receivable	21,331	19,405
Financial liabilities:
Deposits	5,723,916	5,247,604
Borrowed funds	545,089	937,806
Accrued interest payable	7,119	1,037
Other liabilities	4,781	3,188
Book Value | Fair Value Inputs Level 1 And Level 3
Financial assets:
Other investments		14,671
Book Value | Fair Value Inputs Level 2 And Level 3
Financial assets:
Loans, net	4,835,591	4,719,229
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	35,083
Impaired loans, net	50,545	57,089
Other assets	977	2,883
Financial liabilities:
Standby letters of credit	328	603
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash and cash equivalents	711,372	723,866
Fair Value | Fair Value Inputs Level 1 And Level 2
Financial assets:
Investment securities available for sale	997,131	837,499
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	150,583	162,853
Loans, held for sale	35,855	31,055
Stock in FHLB of Pittsburgh	16,540	31,284
Accrued interest receivable	21,331	19,405
Financial liabilities:
Deposits	5,685,435	4,848,588
Borrowed funds	538,395	937,605
Accrued interest payable	7,119	1,037
Other liabilities	4,781	3,188
Fair Value | Fair Value Inputs Level 1 And Level 3
Financial assets:
Other investments		45,326
Fair Value | Fair Value Inputs Level 2 And Level 3
Financial assets:
Loans, net	4,802,841	4,699,458
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	35,083
Impaired loans, net	50,545	57,089
Other assets	977	2,883
Financial liabilities:
Standby letters of credit	$ 328	$ 603