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, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	$ 305,646	$ 254,153
Acquisitions	71,111	115,352
Accretion	54,664	37,078
Reclass from non-accretable difference	9,925	16,393
Disposals/transfers	(5,390)	(592)
Balance at end of period	291,881	305,646
Recorded Investment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	22,192	16,623
Acquisitions	4,340	15,260
Accretion	4,530	3,556
Payments received	(19,123)	(9,325)
Reclass from non-accretable difference		0
Disposals/transfers	(3,663)	(3,922)
Contractual interest		0
Balance at end of period	8,276	22,192
Accretable Yield [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	(7,456)	(778)
Acquisitions	(704)	(2,505)
Accretion	4,530	3,556
Payments received		0
Reclass from non-accretable difference	(2,612)	(8,097)
Disposals/transfers	2,938	368
Contractual interest		0
Balance at end of period	(3,304)	(7,456)
Expected Cash Flows [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	29,648	17,401
Acquisitions	5,044	17,765
Accretion		0
Payments received	(19,123)	(9,325)
Reclass from non-accretable difference	2,612	8,097
Disposals/transfers	(6,601)	(4,290)
Contractual interest		0
Balance at end of period	11,580	29,648
Non-Accretable Difference [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	(26,852)	(23,733)
Acquisitions	(5,426)	(24,266)
Accretion		0
Payments received	1,877	1,345
Reclass from non-accretable difference	2,612	8,097
Disposals/transfers	19,299	14,405
Contractual interest	(2,102)	(2,700)
Balance at end of period	(10,592)	(26,852)
Outstanding Balance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	56,500	41,134
Acquisitions	10,470	42,031
Accretion		0
Payments received	(21,000)	(10,670)
Reclass from non-accretable difference		0
Disposals/transfers	(25,900)	(18,695)
Contractual interest	2,102	2,700
Balance at end of period	$ 22,172	$ 56,500