Loans and Allowances for Loan Losses - Schedule of Credit Quality Indicators of Various Classes of Loans (Detail) (Commercial loans [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	$ 8,291,509	$ 8,345,688
Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	5,161,227	4,927,460
Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	2,996,718	3,265,340
Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	133,564	152,888
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	7,521,484	7,555,133
Pass [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	4,754,597	4,521,932
Pass [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	2,756,779	3,011,320
Pass [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	10,108	21,881
Pass-Watch [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	184,942	175,127
Pass-Watch [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	95,595	82,605
Pass-Watch [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	68,070	71,405
Pass-Watch [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	21,277	21,117
Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	125,724	131,049
Special Mention [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	80,776	83,985
Special Mention [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	38,730	39,631
Special Mention [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	6,218	7,433
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	408,236	430,145
Substandard [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	230,100	238,486
Substandard [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	132,958	142,618
Substandard [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	45,178	49,041
Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	51,123	54,234
Doubtful [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	159	452
Doubtful [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	181	366
Doubtful [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial credit risk profile by internally assigned grade	$ 50,783	$ 53,416