Loans and Allowance for Loan Losses - Schedule of Credit Risk Profile by Internally Assigned Grade (Detail) (Residential mortgage loans [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	$ 1,561,640	$ 1,507,498
Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	757,471	487,147
Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	532,551	734,669
Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	271,618	285,682
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	1,350,075	1,254,192
Pass [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	730,385	460,261
Pass [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	491,450	673,751
Pass [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	128,240	120,180
Pass-Watch [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	22,108	27,405
Pass-Watch [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	3,422	7,499
Pass-Watch [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	3,686	1,773
Pass-Watch [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	15,000	18,133
Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	9,713	13,514
Special Mention [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	784	542
Special Mention [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	5,879	9,686
Special Mention [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	3,050	3,286
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	161,544	207,069
Substandard [Member] | Originated loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	22,880	18,845
Substandard [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	31,480	48,581
Substandard [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	107,184	139,643
Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	18,200	5,318
Doubtful [Member] | Acquired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	56	878
Doubtful [Member] | Covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit risk profile by internally assigned grade	$ 18,144	$ 4,440