Fair Value Disclosures - Book Value and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Investment securities available for sale	$ 838,122	$ 794,543
Investment securities held to maturity	161,186	164,346
Loans, held for sale	31,055	54,782
Loans, net	4,776,318	4,444,375
Impaired loans, net	57,089	46,499
Stock in Federal Home Loan Bank of Pittsburgh	31,284	38,248
Accrued interest receivable	19,405	17,027
Other assets	76,416	84,892
Financial liabilities:
Deposits	5,247,604	4,738,438
Accrued interest payable	1,037	1,151
Other liabilities	88,748	70,898
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Loans, held for sale	0	0
Impaired loans, net	0	0
Significant Other Observable Inputs (Level 2)
Financial assets:
Loans, held for sale	31,055	54,782
Impaired loans, net	0	0
Significant Unobservable Inputs (Level 3)
Financial assets:
Loans, held for sale	0	0
Impaired loans, net	57,089	46,499
Fair Value
Financial assets:
Investment securities available for sale	838,122	794,543
Loans, held for sale	31,055	54,782
Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	723,866	821,923
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	162,853	163,232
Stock in Federal Home Loan Bank of Pittsburgh	31,284	38,248
Accrued interest receivable	19,405	17,027
Financial liabilities:
Deposits	4,848,588	4,423,921
Borrowed funds	937,605	1,264,170
Accrued interest payable	1,037	1,151
Other liabilities	3,188	3,380
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Loans, net	4,699,458	4,300,963
Impaired loans, net	57,089	46,499
Other assets	47,586	15,787
Financial liabilities:
Standby letters of credit	603	468
Book Value
Financial assets:
Investment securities available for sale	838,122	794,543
Loans, held for sale	31,055	54,782
Book Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents	723,866	821,923
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities held to maturity	161,186	164,346
Stock in Federal Home Loan Bank of Pittsburgh	31,284	38,248
Accrued interest receivable	19,405	17,027
Financial liabilities:
Deposits	5,247,604	4,738,438
Borrowed funds	937,806	1,267,447
Accrued interest payable	1,037	1,151
Other liabilities	3,188	3,380
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Loans, net	4,719,229	4,397,876
Impaired loans, net	57,089	46,499
Other assets	16,931	9,189
Financial liabilities:
Standby letters of credit	$ 603	$ 468