Fair Value Measurements - Fair Values of Corporation's Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 234,246	$ 287,393	$ 246,253	$ 213,981
Securities available for sale	1,537,080	1,534,065
Securities held to maturity	1,513,204	1,453,355
Securities held to maturity	1,536,449	1,468,258
Derivative assets	57,551	45,898
Deposits	11,806,043	11,382,208
Short-term borrowings	1,740,500	2,041,658
Long-term borrowings	541,474	541,443
Derivative liabilities	54,431	46,160
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	234,246	287,393
Securities available for sale	1,537,080	1,534,065
Securities held to maturity	1,536,449	1,468,258
Net loans and leases, including loans held for sale	11,119,711	10,956,544
Derivative assets	57,551	45,898
Accrued interest receivable	41,935	40,231
Deposits	11,812,401	11,382,402
Short-term borrowings	1,740,499	2,041,672
Long-term borrowings	541,955	539,007
Derivative liabilities	54,431	46,160
Accrued interest payable	6,511	6,689
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	234,246	287,393
Securities available for sale	1,537,080	1,534,065
Securities held to maturity	1,513,204	1,453,355
Net loans and leases, including loans held for sale	11,280,221	11,127,292
Derivative assets	57,551	45,898
Accrued interest receivable	41,935	40,231
Deposits	11,806,043	11,382,208
Short-term borrowings	1,740,500	2,041,658
Long-term borrowings	541,474	541,443
Derivative liabilities	54,431	46,160
Accrued interest payable	6,511	6,689
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	234,246	287,393
Securities available for sale	212	218
Accrued interest receivable	41,935	40,231
Deposits	9,217,159	8,771,173
Short-term borrowings	1,740,499	2,041,672
Accrued interest payable	6,511	6,689
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	1,535,079	1,531,952
Securities held to maturity	1,532,588	1,463,945
Derivative assets	57,551	45,898
Deposits	2,595,242	2,611,229
Derivative liabilities	54,431	46,160
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	1,789	1,895
Securities held to maturity	3,861	4,313
Net loans and leases, including loans held for sale	11,119,711	10,956,544
Long-term borrowings	$ 541,955	$ 539,007