Loans and the Allowance for Credit Losses - Summary of Commercial Loans by Risk Grade (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Summary of commercial loans by risk grade
Commercial loans	$ 2,519,537	$ 2,469,168
Excellent - Minimal Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	72,513	73,749
Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	933,135	946,815
Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	1,392,422	1,313,020
Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	68,707	75,249
Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	52,760	60,335
Commercial Real Estate - Land and Construction [Member]
Summary of commercial loans by risk grade
Commercial loans	255,499	263,117
Commercial Real Estate - Land and Construction [Member] | Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	23,884	39,409
Commercial Real Estate - Land and Construction [Member] | Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	222,915	213,822
Commercial Real Estate - Land and Construction [Member] | Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	6,282	6,498
Commercial Real Estate - Land and Construction [Member] | Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	2,418	3,388
Commercial Real Estate - Improved Property [Member]
Summary of commercial loans by risk grade
Commercial loans	1,685,373	1,649,802
Commercial Real Estate - Improved Property [Member] | Excellent - Minimal Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	352	360
Commercial Real Estate - Improved Property [Member] | Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	707,407	710,137
Commercial Real Estate - Improved Property [Member] | Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	885,735	838,283
Commercial Real Estate - Improved Property [Member] | Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	50,934	57,983
Commercial Real Estate - Improved Property [Member] | Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	40,945	43,039
Commercial and Industrial [Member]
Summary of commercial loans by risk grade
Commercial loans	578,665	556,249
Commercial and Industrial [Member] | Excellent - Minimal Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	72,161	73,389
Commercial and Industrial [Member] | Good - Desirable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	201,844	197,269
Commercial and Industrial [Member] | Fair - Acceptable Risk [Member]
Summary of commercial loans by risk grade
Commercial loans	283,772	260,915
Commercial and Industrial [Member] | Criticized - Marginal [Member]
Summary of commercial loans by risk grade
Commercial loans	11,491	10,768
Commercial and Industrial [Member] | Classified - Substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	$ 9,397	$ 13,908