Loans - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable
Non-covered loans net of unearned discount and deferred loan fees	$ 2,100,000		$ 2,100,000		$ 9,000,000
Non-covered loans	14,335,063,000		14,335,063,000		13,262,022,000
Sales of loans	0	$ 0	0	$ 0
Non-interest income, increase (decrease)	$ 595,000	(7,700,000)	$ (3,300,000)	(7,800,000)
Combined loan-to-value ratio home equity loan	75.00%		75.00%
TDRs not reported as non-accrual loans	$ 97,600,000		$ 97,600,000		97,700,000
Non-performing TDRs	17,600,000		$ 17,600,000		19,400,000
Specific reserves for loan losses	1,000,000	$ 2,300,000
Partial loan charge-offs related to loans modified as TDRs				$ 861,000
Number of days loans placed on non-accrual status			90 days
Repossessed Assets	19,900,000		$ 19,900,000
Residential real estate properties
Accounts, Notes, Loans and Financing Receivable
Repossessed Assets	6,400,000		6,400,000
In formal foreclosure proceedings
Accounts, Notes, Loans and Financing Receivable
Residential mortgage and consumer loans secured by residential real estate properties	13,500,000		13,500,000
Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Unsecured loans	349,500,000		349,500,000		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	1,500,000,000		1,500,000,000		1,600,000,000
Outstanding contractual principal balances	165,200,000		165,200,000		253,700,000
Consumer loans
Accounts, Notes, Loans and Financing Receivable
Non-covered loans	2,031,613,000		2,031,613,000		1,946,896,000
Unsecured loans	21,500,000		21,500,000		31,400,000
Credit card loans
Accounts, Notes, Loans and Financing Receivable
Unsecured loans	$ 7,200,000		$ 7,200,000		$ 7,600,000