Loans - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Repossessed Assets	$ 21,800,000
Non-covered loans net of unearned discount and deferred loan fees	9,200,000		9,000,000
Non-covered loans	13,550,735,000		13,262,022,000
Sales of loans	0	0
Non-interest income, increase (decrease)	3,900,000	76,000
Reimbursable portion of valuation write-downs on certain covered OREO properties	(355,000)	1,256,000
Combined loan-to-value ratio home equity loan	75.00%
TDRs not reported as non-accrual loans	100,500,000		97,700,000
Non-performing TDRs	18,800,000		19,400,000
Specific reserves for loan losses	800,000	2,100,000
Partial loan charge-offs related to loans modified as TDRs		900,000
Number of days loans placed on non-accrual status	90 days
Contributions to qualified plan	0
Foreclosed Assets Measured At Fair Value	2,400,000
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsecured loans	358,900,000		345,100,000
Commercial Portfolio Segment [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired loans	250,000
PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding contractual principal balances	214,900,000		253,700,000
Non-covered loans	1,600,000,000		1,600,000,000
Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-covered loans	1,967,012,000		1,946,896,000
Unsecured loans	30,300,000		31,400,000
Credit Card [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsecured loans	7,000,000		7,600,000
Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Repossessed Assets	6,700,000
In Formal Foreclosure Proceedings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage and consumer loans secured by residential real estate properties	$ 14,400,000