Loans (Weighted Average Risk Grades for All Commercial Loans b Class) (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) Grade	Dec. 31, 2014 USD ($) Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans | $	$ 4,292,727	$ 3,915,877
Loans | $	$ 720,695	$ 624,888
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.13	6.16
Loans | $	$ 4,120,522	$ 4,055,225
Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.89	5.45
Loans | $	$ 1,758,195	$ 1,773,945
Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.85	6.76
Loans | $	$ 3,572,032	$ 3,290,989
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.01	6.97
Loans | $	$ 720,695	$ 624,888
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	5.88	5.93
Loans | $	$ 3,869,203	$ 3,846,598
Risk Grade One To Eight [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.12	5.37
Loans | $	$ 1,385,749	$ 1,740,455
Risk Grade One To Eight [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.58	6.53
Loans | $	$ 3,280,435	$ 3,067,219
Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.91	6.91
Loans | $	$ 696,229	$ 612,705
Risk Grade Nine [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans | $	$ 100,670	$ 65,166
Risk Grade Nine [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans | $	$ 212,250	$ 27,313
Risk Grade Nine [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 | $	$ 140,900	$ 72,906
Risk Grade Nine [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans | $	$ 13,074	$ 8,003
Risk Grade Ten [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans | $	$ 76,030	$ 54,519
Risk Grade Ten [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans | $	$ 62,163	$ 161
Risk Grade Ten [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 | $	$ 72,577	$ 87,889
Risk Grade Ten [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans | $	$ 2,757	$ 1,323
Risk Grade Eleven [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans | $	$ 49,508	$ 55,034
Risk Grade Eleven [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans | $	$ 76,853	$ 5,380
Risk Grade Eleven [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 | $	$ 43,601	$ 43,336
Risk Grade Eleven [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans | $	$ 8,066	$ 64
Risk Grade Twelve [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans | $	$ 22,644	$ 31,683
Risk Grade Twelve [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans | $	$ 19,180	$ 636
Risk Grade Twelve [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 | $	$ 34,519	$ 19,501
Risk Grade Twelve [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans | $	$ 569	$ 2,793
Risk Grade Thirteen [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans | $	$ 2,467	$ 2,225
Risk Grade Thirteen [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans | $	$ 2,000	$ 0
Risk Grade Thirteen [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	$ 138
Risk Grade Thirteen [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans | $	$ 0	$ 0
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	1.00	1.00
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	8.00	8.00