Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) Grade	Dec. 31, 2015 USD ($) Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,386,803	$ 4,292,727
Loans	$ 752,916	$ 720,695
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.21	6.13
Loans	$ 4,184,960	$ 4,120,522
Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	8.18	6.89
Loans	$ 1,656,360	$ 1,758,195
Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.89	6.85
Loans	$ 3,633,887	$ 3,572,032
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.09	7.01
Loans	$ 752,916	$ 720,695
Risk grades 1-8 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	5.92	5.88
Loans	$ 3,888,217	$ 3,869,203
Risk grades 1-8 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.16	6.12
Loans	$ 846,191	$ 1,385,749
Risk grades 1-8 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.60	6.58
Loans	$ 3,324,030	$ 3,280,435
Risk grades 1-8 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.05	6.91
Loans	$ 739,863	$ 696,229
Risk grade 9 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 121,570	$ 100,670
Risk grade 9 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 216,140	$ 212,250
Risk grade 9 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 133,790	$ 140,900
Risk grade 9 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans	$ 10,759	$ 13,074
Risk grade 10 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 84,614	$ 76,030
Risk grade 10 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 276,693	$ 62,163
Risk grade 10 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 79,584	$ 72,577
Risk grade 10 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans	$ 1,271	$ 2,757
Risk grade 11 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 63,637	$ 49,508
Risk grade 11 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 203,213	$ 76,853
Risk grade 11 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 63,034	$ 43,601
Risk grade 11 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans	$ 53	$ 8,066
Risk grade 12 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 23,994	$ 22,644
Risk grade 12 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 85,919	$ 19,180
Risk grade 12 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 33,449	$ 34,519
Risk grade 12 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans	$ 970	$ 569
Risk grade 13 [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 2,928	$ 2,467
Risk grade 13 [Member] | Commercial and industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 28,204	$ 2,000
Risk grade 13 [Member] | Commercial real estate, Buildings, land and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 0	$ 0
Risk grade 13 [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans	$ 0	$ 0