Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) (USD $) In Thousands, unless otherwise specified				9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Sep. 30, 2014 Commercial and industrial, Energy [Member] Grade	Dec. 31, 2013 Commercial and industrial, Energy [Member] Grade	Sep. 30, 2014 Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2013 Commercial and industrial, Other commercial [Member] Grade	Sep. 30, 2014 Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2013 Commercial real estate, Buildings, land and other [Member] Grade	Sep. 30, 2014 Commercial real estate, Construction [Member] Grade	Dec. 31, 2013 Commercial real estate, Construction [Member] Grade	Sep. 30, 2014 Risk grades 1-8 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2013 Risk grades 1-8 [Member] Commercial and industrial, Energy [Member] Grade	Sep. 30, 2014 Risk grades 1-8 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2013 Risk grades 1-8 [Member] Commercial and industrial, Other commercial [Member] Grade	Sep. 30, 2014 Risk grades 1-8 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2013 Risk grades 1-8 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Sep. 30, 2014 Risk grades 1-8 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2013 Risk grades 1-8 [Member] Commercial real estate, Construction [Member] Grade	Sep. 30, 2014 Risk grade 9 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2013 Risk grade 9 [Member] Commercial and industrial, Energy [Member] Grade	Sep. 30, 2014 Risk grade 9 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2013 Risk grade 9 [Member] Commercial and industrial, Other commercial [Member] Grade	Sep. 30, 2014 Risk grade 9 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2013 Risk grade 9 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Sep. 30, 2014 Risk grade 9 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2013 Risk grade 9 [Member] Commercial real estate, Construction [Member] Grade	Sep. 30, 2014 Risk grade 10 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2013 Risk grade 10 [Member] Commercial and industrial, Energy [Member] Grade	Sep. 30, 2014 Risk grade 10 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2013 Risk grade 10 [Member] Commercial and industrial, Other commercial [Member] Grade	Sep. 30, 2014 Risk grade 10 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2013 Risk grade 10 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Sep. 30, 2014 Risk grade 10 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2013 Risk grade 10 [Member] Commercial real estate, Construction [Member] Grade	Sep. 30, 2014 Risk grade 11 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2013 Risk grade 11 [Member] Commercial and industrial, Energy [Member] Grade	Sep. 30, 2014 Risk grade 11 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2013 Risk grade 11 [Member] Commercial and industrial, Other commercial [Member] Grade	Sep. 30, 2014 Risk grade 11 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2013 Risk grade 11 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Sep. 30, 2014 Risk grade 11 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2013 Risk grade 11 [Member] Commercial real estate, Construction [Member] Grade	Sep. 30, 2014 Risk grade 12 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2013 Risk grade 12 [Member] Commercial and industrial, Energy [Member] Grade	Sep. 30, 2014 Risk grade 12 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2013 Risk grade 12 [Member] Commercial and industrial, Other commercial [Member] Grade	Sep. 30, 2014 Risk grade 12 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2013 Risk grade 12 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Sep. 30, 2014 Risk grade 12 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2013 Risk grade 12 [Member] Commercial real estate, Construction [Member] Grade	Sep. 30, 2014 Risk grade 13 [Member] Commercial and industrial, Energy [Member] Grade	Dec. 31, 2013 Risk grade 13 [Member] Commercial and industrial, Energy [Member] Grade	Sep. 30, 2014 Risk grade 13 [Member] Commercial and industrial, Other commercial [Member] Grade	Dec. 31, 2013 Risk grade 13 [Member] Commercial and industrial, Other commercial [Member] Grade	Sep. 30, 2014 Risk grade 13 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Dec. 31, 2013 Risk grade 13 [Member] Commercial real estate, Buildings, land and other [Member] Grade	Sep. 30, 2014 Risk grade 13 [Member] Commercial real estate, Construction [Member] Grade	Dec. 31, 2013 Risk grade 13 [Member] Commercial real estate, Construction [Member] Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade				5.38	5.40	6.18	6.27	6.73	6.83	7.05	7.10	5.29	5.37	5.94	5.95	6.51	6.59	7.00	7.05	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	$ 5,510,887	$ 4,907,076	$ 4,808,672	$ 1,600,466	$ 1,115,409	$ 3,910,421	$ 3,791,667					$ 1,566,621	$ 1,106,348	$ 3,697,503	$ 3,507,963					$ 24,212	$ 7,726	$ 58,045	$ 74,766					$ 3,161	$ 245	$ 65,561	$ 89,878					$ 5,835	$ 500	$ 55,403	$ 92,917					$ 637	$ 590	$ 32,008	$ 21,389					$ 0	$ 0	$ 1,901	$ 4,754
Loans	4,020,478	3,467,336	3,370,969					3,377,486	3,040,697							3,164,357	2,844,665							80,878	65,770							70,312	49,881							41,549	53,208							19,614	24,387							776	2,786
Loans	$ 642,992	$ 426,639								$ 642,992	$ 426,639							$ 631,616	$ 418,999							$ 2,494	$ 1,301							$ 5,855	$ 5,931							$ 2,738	$ 408							$ 289	$ 0							$ 0	$ 0