Loans and the Allowance for Credit Losses - Summary of Commercial Loans by Risk Grade (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of commercial loans by risk grade
Commercial loans	$ 2,423,154	$ 2,336,370
Excellent - Minimal Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	70,343	65,044
Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	917,703	892,592
Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	1,276,030	1,205,997
Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	78,457	86,777
Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	80,621	85,960
Classified - Doubtful [Member]
Summary of commercial loans by risk grade
Commercial loans
Commercial Real Estate - Land and Construction [Member]
Summary of commercial loans by risk grade
Commercial loans	195,498	193,004
Commercial Real Estate - Land and Construction [Member] | Excellent - Minimal Risk [Member]
Summary of commercial loans by risk grade
Commercial loans
Commercial Real Estate - Land and Construction [Member] | Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	27,766	38,292
Commercial Real Estate - Land and Construction [Member] | Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	151,735	136,643
Commercial Real Estate - Land and Construction [Member] | Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	7,994	10,573
Commercial Real Estate - Land and Construction [Member] | Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	8,003	7,496
Commercial Real Estate - Land and Construction [Member] | Classified - Doubtful [Member]
Summary of commercial loans by risk grade
Commercial loans
Commercial Real Estate - Improved Property [Member]
Summary of commercial loans by risk grade
Commercial loans	1,685,585	1,665,341
Commercial Real Estate - Improved Property [Member] | Excellent - Minimal Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	704	789
Commercial Real Estate - Improved Property [Member] | Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	710,947	701,447
Commercial Real Estate - Improved Property [Member] | Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	851,520	826,790
Commercial Real Estate - Improved Property [Member] | Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	58,704	66,906
Commercial Real Estate - Improved Property [Member] | Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	63,710	69,409
Commercial Real Estate - Improved Property [Member] | Classified - Doubtful [Member]
Summary of commercial loans by risk grade
Commercial loans
Commercial and Industrial [Member]
Summary of commercial loans by risk grade
Commercial loans	542,071	478,025
Commercial and Industrial [Member] | Excellent - Minimal Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	69,639	64,255
Commercial and Industrial [Member] | Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	178,990	152,853
Commercial and Industrial [Member] | Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	272,775	242,564
Commercial and Industrial [Member] | Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	11,759	9,298
Commercial and Industrial [Member] | Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	8,908	9,055
Commercial and Industrial [Member] | Classified - Doubtful [Member]
Summary of commercial loans by risk grade
Commercial loans