Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) (USD $) In Thousands, unless otherwise specified				6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Jun. 30, 2013 Commercial and industrial, Energy [Member] Grade	Dec. 31, 2012 Commercial and industrial, Energy [Member] Grade	Jun. 30, 2013 Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2012 Commercial and industrial, Other commercial [Member] Grade	Jun. 30, 2013 Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2012 Commercial real estate, Buildings, land and other [Member] Grade	Jun. 30, 2013 Commercial real estate, Construction [Member] Grade	Dec. 31, 2012 Commercial real estate, Construction [Member] Grade	Jun. 30, 2013 Risk grades 1-8 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2012 Risk grades 1-8 [Member] Commercial and industrial, Energy [Member] Grade	Jun. 30, 2013 Risk grades 1-8 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2012 Risk grades 1-8 [Member] Commercial and industrial, Other commercial [Member] Grade	Jun. 30, 2013 Risk grades 1-8 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2012 Risk grades 1-8 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Jun. 30, 2013 Risk grades 1-8 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2012 Risk grades 1-8 [Member] Commercial real estate, Construction [Member] Grade	Jun. 30, 2013 Risk grade 9 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2012 Risk grade 9 [Member] Commercial and industrial, Energy [Member] Grade	Jun. 30, 2013 Risk grade 9 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2012 Risk grade 9 [Member] Commercial and industrial, Other commercial [Member] Grade	Jun. 30, 2013 Risk grade 9 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2012 Risk grade 9 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Jun. 30, 2013 Risk grade 9 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2012 Risk grade 9 [Member] Commercial real estate, Construction [Member] Grade	Jun. 30, 2013 Risk grade 10 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2012 Risk grade 10 [Member] Commercial and industrial, Energy [Member] Grade	Jun. 30, 2013 Risk grade 10 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2012 Risk grade 10 [Member] Commercial and industrial, Other commercial [Member] Grade	Jun. 30, 2013 Risk grade 10 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2012 Risk grade 10 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Jun. 30, 2013 Risk grade 10 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2012 Risk grade 10 [Member] Commercial real estate, Construction [Member] Grade	Jun. 30, 2013 Risk grade 11 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2012 Risk grade 11 [Member] Commercial and industrial, Energy [Member] Grade	Jun. 30, 2013 Risk grade 11 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2012 Risk grade 11 [Member] Commercial and industrial, Other commercial [Member] Grade	Jun. 30, 2013 Risk grade 11 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2012 Risk grade 11 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Jun. 30, 2013 Risk grade 11 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2012 Risk grade 11 [Member] Commercial real estate, Construction [Member] Grade	Jun. 30, 2013 Risk grade 12 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2012 Risk grade 12 [Member] Commercial and industrial, Energy [Member] Grade	Jun. 30, 2013 Risk grade 12 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2012 Risk grade 12 [Member] Commercial and industrial, Other commercial [Member] Grade	Jun. 30, 2013 Risk grade 12 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2012 Risk grade 12 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Jun. 30, 2013 Risk grade 12 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2012 Risk grade 12 [Member] Commercial real estate, Construction [Member] Grade	Jun. 30, 2013 Risk grade 13 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2012 Risk grade 13 [Member] Commercial and industrial, Energy [Member] Grade	Jun. 30, 2013 Risk grade 13 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2012 Risk grade 13 [Member] Commercial and industrial, Other commercial [Member] Grade	Jun. 30, 2013 Risk grade 13 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2012 Risk grade 13 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Jun. 30, 2013 Risk grade 13 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2012 Risk grade 13 [Member] Commercial real estate, Construction [Member] Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade				5.31	5.25	6.21	6.21	6.86	6.97	6.95	6.94	5.28	5.24	5.91	5.81	6.56	6.63	6.85	6.82	9.00	9.00	9.00	9.00	9.00	9.00	9.00	9.00	10.00	10.00	10.00	10.00	10.00	10.00	10.00	10.00	11.00	11.00	11.00	11.00	11.00	11.00	11.00	11.00	12.00	12.00	12.00	12.00	12.00	12.00	12.00	12.00	13.00	13.00	13.00	13.00	13.00	13.00	13.00	13.00
Loans	$ 4,772,617	$ 4,828,612	$ 4,309,889	$ 1,017,337	$ 1,083,186	$ 3,755,280	$ 3,745,426					$ 1,009,557	$ 1,081,725	$ 3,481,071	$ 3,367,443					$ 7,395	$ 392	$ 148,192	$ 250,508							$ 28,187	$ 28,440					$ 385		$ 56,811	$ 53,797						$ 169	$ 36,070	$ 40,603						$ 900	$ 4,949	$ 4,635
Loans	3,371,268	3,319,795	3,119,902					2,782,525	2,711,489							2,564,596	2,460,448							74,938	92,041							38,397	42,603							62,929	77,658							38,321	35,602							3,344	3,137
Loans	$ 588,743	$ 608,306								$ 588,743	$ 608,306							$ 563,561	$ 579,108							$ 20,977	$ 23,046							$ 2,490	$ 4,435							$ 700	$ 617							$ 1,015	$ 1,100