Fair Value Measurements - Fair Values of Corporation's Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 399,027	$ 489,119	$ 237,479	$ 287,393
Securities available for sale	2,133,662	1,630,567
Securities held to maturity	2,064,305	1,637,061
Securities held to maturity	2,104,782	1,643,416
Derivative assets	108,149	53,202
Deposits	15,528,562	12,623,463
Short-term borrowings	2,260,411	2,048,896
Long-term borrowings	656,844	641,480
Derivative liabilities	100,817	51,605
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	399,027	489,119
Securities available for sale	2,133,662	1,630,567
Securities held to maturity	2,104,782	1,643,416
Net loans and leases, including loans held for sale	14,398,390	11,863,882
Derivative assets	108,149	53,202
Accrued interest receivable	53,713	44,920
Deposits	15,540,205	12,610,914
Short-term borrowings	2,259,874	2,048,943
Long-term borrowings	657,812	637,935
Derivative liabilities	100,817	51,605
Accrued interest payable	7,631	7,457
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	399,027	489,119
Securities available for sale	2,133,662	1,630,567
Securities held to maturity	2,064,305	1,637,061
Net loans and leases, including loans held for sale	14,420,821	12,053,209
Derivative assets	108,149	53,202
Accrued interest receivable	53,713	44,920
Deposits	15,528,562	12,623,463
Short-term borrowings	2,260,411	2,048,896
Long-term borrowings	656,844	641,480
Derivative liabilities	100,817	51,605
Accrued interest payable	7,631	7,457
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	399,027	489,119
Securities available for sale	255	225
Accrued interest receivable	53,713	44,920
Deposits	12,910,925	10,157,997
Short-term borrowings	2,259,874	2,048,943
Accrued interest payable	7,631	7,457
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	2,132,005	1,628,719
Securities held to maturity	2,102,638	1,640,721
Derivative assets	108,149	53,202
Deposits	2,629,280	2,452,917
Derivative liabilities	100,817	51,605
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	1,402	1,623
Securities held to maturity	2,144	2,695
Net loans and leases, including loans held for sale	14,398,390	11,863,882
Long-term borrowings	$ 657,812	$ 637,935