Loans - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2014 USD ($)	Jun. 30, 2013	Dec. 31, 2015 USD ($) Contract	Dec. 31, 2014 USD ($) SecurityLoan	Dec. 31, 2014 USD ($)	Dec. 31, 2014 USD ($) Contract	Dec. 31, 2014 USD ($) Borrower_modified	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-covered loans net of unearned discount and deferred loan fees			$ 3,500,000	$ 9,000,000	$ 9,000,000	$ 9,000,000	$ 9,000,000
Loans			16,043,107,000	13,473,913,000	13,473,913,000	13,473,913,000	13,473,913,000
Nonperforming loans	$ 35,600,000
Loan charge-offs	$ 8,300,000
Sales of loans			0		0
FDIC loss-share receivable			8,300,000	13,800,000	13,800,000	13,800,000	13,800,000
Non-interest income, increase (decrease)			$ 3,300,000		20,800,000			$ 8,400,000
Average expected life of the non-covered PCI loans		2 years 2 months 12 days
Previous average expected life of the non-covered PCI loans		4 years
Combined loan-to-value ratio home equity loan			75.00%
Accrued interest on non-accrual loans			$ 3,500,000		2,200,000			4,800,000
Interest income recognized and classified as non-accrual			1,300,000		735,000			$ 1,300,000
TDRs not reported as non-accrual loans			77,600,000	97,700,000	97,700,000	97,700,000	97,700,000
Non-performing TDRs			21,000,000	19,400,000	19,400,000	$ 19,400,000	19,400,000
Specific reserves for loan losses			$ 1,400,000		3,600,000
Number of loans modified | Contract			33			42
Troubled debt restructuring, charge-offs			$ 320,000		861,000
Number of days loans placed on non-accrual status			90 days
PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans			$ 2,400,000,000	1,900,000,000	1,900,000,000	$ 1,900,000,000	1,900,000,000
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsecured loans			7,100,000	7,600,000	7,600,000	7,600,000	7,600,000
Covered by FDIC Loss Share [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-interest income, increase (decrease)			(57,100,000)		60,100,000
Commercial and industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans			2,540,491,000	2,251,111,000	2,251,111,000	2,251,111,000	2,251,111,000
Unsecured loans			386,600,000	$ 345,100,000	345,100,000	$ 345,100,000	$ 345,100,000
Impaired loans			$ 250,000
Number of loans modified			17	2		12
Troubled debt restructuring, charge-offs					6,100,000
Number of borrowers whose loans were modified | Borrower_modified							1
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans			$ 2,192,492,000	$ 1,952,415,000	1,952,415,000	$ 1,952,415,000	$ 1,952,415,000
Unsecured loans			$ 18,800,000	$ 31,400,000	$ 31,400,000	$ 31,400,000	$ 31,400,000
Number of loans modified | Contract			2			5