Fair Value Disclosures of Financial Assets and Liabilities - Book Value and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial assets:
Investment securities, available for sale	$ 2,529,057	$ 1,944,914
Investment securities, held to maturity, net	111,747	133,601
Stock in FHLB of Pittsburgh	5,771	21,097
Accrued interest receivable	44,335	38,094
Financial liabilities:
Deposits	11,856,664	9,586,857
Accrued interest payable	1,450	3,103
Fair Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash, cash equivalents and restricted cash	1,654,735	571,752
Fair Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities, available for sale	2,529,057	1,944,914
Investment securities, held to maturity, net	116,421	136,625
Loans, held for sale	197,541	83,872
Stock in FHLB of Pittsburgh	5,771	21,097
Accrued interest receivable	44,335	38,094
Other assets	14,448	4,884
Financial liabilities:
Deposits	11,868,897	9,575,394
Borrowed funds	340,106	489,561
Accrued interest payable	1,450	3,103
Other liabilities	32,257	3,918
Fair Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	9,541	70,046
Loans, net	9,130,003	8,580,015
Financial liabilities:
Standby letters of credit	630	623
Other liabilities	32,257	3,918
Book Value | Quoted Prices in Active Markets for Identical Asset (Level 1)
Financial assets:
Cash, cash equivalents and restricted cash	1,654,735	571,752
Book Value | Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities, available for sale	2,529,057	1,944,914
Investment securities, held to maturity, net	111,741	133,601
Loans, held for sale	197,541	83,872
Stock in FHLB of Pittsburgh	5,771	21,097
Accrued interest receivable	44,335	38,094
Other assets	14,448	4,884
Financial liabilities:
Deposits	11,856,664	9,586,857
Borrowed funds	340,641	489,288
Accrued interest payable	1,450	3,103
Other liabilities	32,257	3,918
Book Value | Significant Unobservable Inputs (Level 3)
Financial assets:
Other investments	9,541	70,046
Loans, net	8,795,935	8,424,464
Financial liabilities:
Standby letters of credit	630	623
Other liabilities	$ 32,257	$ 3,918