Loans - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable
Non-covered loans net of unearned premiums, discounts and deferred loan fees	$ 106,000		$ 106,000		$ 9,000,000
Non-covered loans	14,887,323,000		14,887,323,000		13,262,022,000
Sales of loans	0	$ 0	0	$ 0
Non-interest income, increase (decrease)	$ (55,000)	(3,800,000)	$ (3,400,000)	(11,600,000)
Combined loan-to-value ratio home equity loan	75.00%		75.00%
TDRs not reported as non-accrual loans	$ 91,200,000		$ 91,200,000		97,700,000
Non-performing TDRs	14,800,000		14,800,000		19,400,000
Specific reserves for loan losses	602,000	3,800,000
Partial loan charge-offs related to loans modified as TDRs	0	$ 0	$ 0	$ 861,000
Number of days loans placed on non-accrual status			90 days
Repossessed Assets	20,100,000		$ 20,100,000
Residential real estate properties
Accounts, Notes, Loans and Financing Receivable
Repossessed Assets	5,800,000		5,800,000
In formal foreclosure proceedings
Accounts, Notes, Loans and Financing Receivable
Residential mortgage and consumer loans secured by residential real estate properties	12,300,000		12,300,000
Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Unsecured loans	360,800,000		360,800,000		345,100,000
Commercial real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Impaired loans	250,000		250,000
PCI Loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans	1,400,000,000		1,400,000,000		1,600,000,000
Outstanding contractual principal balances	149,300,000		149,300,000		253,700,000
Consumer loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans	2,083,193,000		2,083,193,000		1,946,896,000
Unsecured loans	17,400,000		17,400,000		31,400,000
Credit card loans
Accounts, Notes, Loans and Financing Receivable
Unsecured loans	$ 7,000,000		$ 7,000,000		$ 7,600,000