Fair Value Measurements - Fair Values of Corporation's Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 287,393	$ 213,981	$ 239,044	$ 208,953
Securities available for sale	1,534,065	1,141,650	1,172,683
Securities held to maturity	1,453,355	1,199,169	1,106,563
Securities held to maturity	1,468,258	1,189,563	1,143,213
Derivative assets	45,898	33,317
Deposits	11,382,208	10,198,232
Short-term borrowings	2,041,658	1,241,239
Long-term debt	483,197	143,928
Junior subordinated debt	58,246	75,205
Derivative liabilities	46,160	43,369
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	287,393	213,981
Securities available for sale	1,534,065	1,141,650
Securities held to maturity	1,468,258	1,189,563
Net loans, including loans held for sale	10,956,544	9,243,780
Derivative assets	45,898	33,317
Accrued interest receivable	40,231	35,520
Deposits	11,382,402	10,208,268
Short-term borrowings	2,041,672	1,241,239
Long-term debt	485,517	145,995
Junior subordinated debt	53,490	70,442
Derivative liabilities	46,160	43,369
Accrued interest payable	6,689	7,061
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	287,393	213,981
Securities available for sale	1,534,065	1,141,650
Securities held to maturity	1,453,355	1,199,169
Net loans, including loans held for sale	11,127,292	9,402,448
Derivative assets	45,898	33,317
Accrued interest receivable	40,231	35,520
Deposits	11,382,208	10,198,232
Short-term borrowings	2,041,658	1,241,239
Long-term debt	483,197	143,928
Junior subordinated debt	58,246	75,205
Derivative liabilities	46,160	43,369
Accrued interest payable	6,689	7,061
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	287,393	213,981
Securities available for sale	218	649
Accrued interest receivable	40,231	35,520
Deposits	8,771,173	7,592,159
Short-term borrowings	2,041,672	1,241,239
Accrued interest payable	6,689	7,061
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	1,531,952	1,107,252
Securities held to maturity	1,463,945	1,182,671
Derivative assets	45,898	33,317
Deposits	2,611,229	2,616,109
Derivative liabilities	46,160	43,369
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available for sale	1,895	33,749
Securities held to maturity	4,313	6,892
Net loans, including loans held for sale	10,956,544	9,243,780
Long-term debt	485,517	145,995
Junior subordinated debt	$ 53,490	$ 70,442