Fair Value Disclosures of Financial Assets and Liabilities - Book Value and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Financial assets:
Cash and cash equivalents	$ 529,037	$ 508,039	$ 471,301	$ 484,426
Fair Value	756,731	740,124
Investment securities held-to-maturity	138,060	126,168
Loans, held-for-sale	33,979	28,508
Loans, net	3,324,020	3,156,652
Reverse mortgage loans	24,476	29,298
Stock in FHLB of Pittsburgh	27,943	23,278
Accrued interest receivable	11,770	11,782
Other assets	49,931	54,561
Financial liabilities:
Deposits	3,644,602	3,649,235
Accrued interest payable	1,760	1,004
Significant Other Observable Inputs (Level 2) [Member]
Financial assets:
Loans, held-for-sale	33,979	28,508
Book Value [Member]
Financial assets:
Fair Value	756,731	740,124
Loans, held-for-sale	33,979	28,508
Book Value [Member] | Quoted Prices in Active Markets for Identical Asset (Level 1) [Member]
Financial assets:
Cash and cash equivalents	529,037	508,039
Book Value [Member] | Significant Other Observable Inputs (Level 2) [Member]
Financial assets:
Investment securities held-to-maturity	138,060	126,168
Loans, net	3,324,020	3,156,652
Stock in FHLB of Pittsburgh	27,943	23,278
Accrued interest receivable	11,770	11,782
Financial liabilities:
Deposits	3,644,602	3,649,235
Borrowed funds	868,379	667,775
Accrued interest payable	1,760	1,004
Book Value [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Reverse mortgage loans	24,476	29,298
Financial liabilities:
Standby letters of credit	191	151
Fair Value [Member]
Financial assets:
Fair Value	756,731	740,124
Loans, held-for-sale	33,979	28,508
Fair Value [Member] | Quoted Prices in Active Markets for Identical Asset (Level 1) [Member]
Financial assets:
Cash and cash equivalents	529,037	508,039
Fair Value [Member] | Significant Other Observable Inputs (Level 2) [Member]
Financial assets:
Investment securities held-to-maturity	138,138	126,171
Loans, net	3,281,986	3,121,855
Stock in FHLB of Pittsburgh	27,943	23,278
Accrued interest receivable	11,770	11,782
Financial liabilities:
Deposits	3,464,311	3,461,218
Borrowed funds	871,082	672,850
Accrued interest payable	1,760	1,004
Fair Value [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Reverse mortgage loans	24,476	29,298
Other assets	5,837	4,837
Financial liabilities:
Standby letters of credit	$ 191	$ 151