Loans and Allowance for Loan Losses - Summary of Information about Corporation's Purchased Credit-Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period, Accretable Yield	$ (254,153)	$ (51,706)
Accretion	29,879	47,686
Reclass from non-accretable difference	7,873	14,454
Disposals/transfers	(46)	(664)
Balance at end of period, Accretable Yield	(236,399)	(254,153)
Recorded Investment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	16,623	15,839
Acquisitions	3,848	5,565
Accretion	2,250	5,778
Payments received	(3,087)	(9,556)
Disposals/transfers	(2,413)	(1,003)
Balance at end of period	17,221	16,623
Accretable Yield [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period, Accretable Yield	(778)	(2,477)
Acquisitions, Accretable Yield	(523)	(589)
Accretion	2,250	5,778
Payments received, Accretable Yield
Reclass from non-accretable difference	(6,318)	(3,539)
Disposals/transfers	(164)	49
Contractual interest
Balance at end of period, Accretable Yield	(5,533)	(778)
Expected Cash Flows [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	17,401	18,316
Acquisitions	4,371	6,154
Accretion
Payments received	(3,087)	(9,556)
Reclass from non-accretable difference	6,318	3,539
Disposals/transfers	(2,249)	(1,052)
Contractual interest
Balance at end of period	22,754	17,401
Non-Accretable Difference [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	(23,733)	(33,377)
Acquisitions	(7,849)	(2,981)
Accretion
Payments received
Reclass from non-accretable difference	6,318	3,539
Disposals/transfers	6,193	11,442
Contractual interest	(1,942)	(2,356)
Balance at end of period	(21,013)	(23,733)
Outstanding Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	41,134	51,693
Acquisitions	12,220	9,135
Accretion
Payments received	(3,087)	(9,556)
Reclass from non-accretable difference
Disposals/transfers	(8,442)	(12,494)
Contractual interest	1,942	2,356
Balance at end of period	$ 43,767	$ 41,134