Fair Value Measurements - Fair Values of Corporation's Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 237,479	$ 287,393	$ 270,720	$ 213,981
Securities available for sale	1,618,620	1,534,065
Securities held to maturity	1,518,060	1,453,355
Securities held to maturity	1,526,040	1,468,258
Derivative assets	43,049	45,898
Deposits	12,358,127	11,382,208
Short-term borrowings	1,507,582	2,041,658
Long-term borrowings	542,578	541,443
Derivative liabilities	43,319	46,160
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	237,479	287,393
Securities available for sale	1,618,620	1,534,065
Securities held to maturity	1,526,040	1,468,258
Net loans and leases, including loans held for sale	11,312,391	10,956,544
Derivative assets	43,049	45,898
Accrued interest receivable	41,824	40,231
Deposits	12,356,232	11,382,402
Short-term borrowings	1,507,582	2,041,672
Long-term borrowings	541,578	539,007
Derivative liabilities	43,319	46,160
Accrued interest payable	6,197	6,689
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	237,479	287,393
Securities available for sale	1,618,620	1,534,065
Securities held to maturity	1,518,060	1,453,355
Net loans and leases, including loans held for sale	11,502,357	11,127,292
Derivative assets	43,049	45,898
Accrued interest receivable	41,824	40,231
Deposits	12,358,127	11,382,208
Short-term borrowings	1,507,582	2,041,658
Long-term borrowings	542,578	541,443
Derivative liabilities	43,319	46,160
Accrued interest payable	6,197	6,689
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	237,479	287,393
Securities available for sale	216	218
Accrued interest receivable	41,824	40,231
Deposits	9,770,550	8,771,173
Short-term borrowings	1,507,582	2,041,672
Accrued interest payable	6,197	6,689
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	1,616,747	1,531,952
Securities held to maturity	1,522,654	1,463,945
Derivative assets	43,049	45,898
Deposits	2,585,682	2,611,229
Derivative liabilities	43,319	46,160
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	1,657	1,895
Securities held to maturity	3,386	4,313
Net loans and leases, including loans held for sale	11,312,391	10,956,544
Long-term borrowings	$ 541,578	$ 539,007