Loans and Allowance for Loan Losses - Schedule of Credit Quality Indicators of Various Classes of Loans (Detail) (Commercial loans [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Grade:
Commercial credit risk profile by internally assigned grade	$ 8,187,469	$ 8,062,158
Originated loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	4,465,736	3,325,972
Acquired loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	3,545,009	4,496,644
Covered loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	176,724	239,542
Pass [Member]
Grade:
Commercial credit risk profile by internally assigned grade	7,297,572	7,010,406
Pass [Member] | Originated loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	4,027,383	3,019,100
Pass [Member] | Acquired loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	3,239,019	3,974,463
Pass [Member] | Covered loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	31,170	16,843
Pass-Watch [Member]
Grade:
Commercial credit risk profile by internally assigned grade	182,505	150,041
Pass-Watch [Member] | Originated loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	85,989	76,393
Pass-Watch [Member] | Acquired loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	76,231	60,042
Pass-Watch [Member] | Covered loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	20,285	13,606
Special Mention [Member]
Grade:
Commercial credit risk profile by internally assigned grade	129,304	170,375
Special Mention [Member] | Originated loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	79,617	35,155
Special Mention [Member] | Acquired loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	42,967	125,852
Special Mention [Member] | Covered loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	6,720	9,368
Substandard [Member]
Grade:
Commercial credit risk profile by internally assigned grade	514,485	653,628
Substandard [Member] | Originated loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	272,267	194,900
Substandard [Member] | Acquired loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	186,541	334,357
Substandard [Member] | Covered loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	55,677	124,371
Doubtful [Member]
Grade:
Commercial credit risk profile by internally assigned grade	63,596	77,596
Doubtful [Member] | Originated loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	480	424
Doubtful [Member] | Acquired loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	251	1,930
Doubtful [Member] | Covered loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	62,865	75,242
Loss [Member]
Grade:
Commercial credit risk profile by internally assigned grade	7	112
Loss [Member] | Covered loans [Member]
Grade:
Commercial credit risk profile by internally assigned grade	$ 7	$ 112