Loans (Weighted Average Risk Grades for All Commercial Loans b Class) (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Grade	Dec. 31, 2018 USD ($) Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 14,750,332	$ 14,099,733
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.44	6.30
Loans, net of unearned discounts	$ 5,187,466	$ 5,111,957
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.39	6.22
Loans, net of unearned discounts	$ 1,652,882	$ 1,602,598
Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.01	6.98
Loans, net of unearned discounts	$ 4,883,580	$ 4,428,721
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.31	7.29
Loans, net of unearned discounts	$ 1,312,659	$ 1,267,717
Risk Grade One To Eight [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.17	6.12
Loans, net of unearned discounts	$ 4,788,857	$ 4,862,275
Risk Grade One To Eight [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	5.90	5.76
Loans, net of unearned discounts	$ 1,488,301	$ 1,451,673
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	6.78	6.76
Loans, net of unearned discounts	$ 4,523,271	$ 4,143,264
Risk Grade One To Eight [Member] | Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.25	7.13
Loans, net of unearned discounts	$ 1,274,098	$ 1,177,260
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, net of unearned discounts	$ 247,212	$ 112,431
Risk Grade Nine [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	9.00	9.00
Loans, net of unearned discounts	$ 32,163	$ 35,565
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, net of unearned discounts	$ 163,714	$ 109,660
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, net of unearned discounts	$ 21,509	$ 60,754
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, net of unearned discounts	$ 71,472	$ 58,328
Risk Grade Ten [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	10.00	10.00
Loans, net of unearned discounts	$ 51,898	$ 43,001
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, net of unearned discounts	$ 103,626	$ 62,353
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, net of unearned discounts	$ 15,243	$ 24,877
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, net of unearned discounts	$ 53,887	$ 69,684
Risk Grade Eleven [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	11.00	11.00
Loans, net of unearned discounts	$ 14,760	$ 25,427
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, net of unearned discounts	$ 84,057	$ 98,176
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, net of unearned discounts	$ 1,144	$ 4,826
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, net of unearned discounts	$ 18,189	$ 6,681
Risk Grade Twelve [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	12.00	12.00
Loans, net of unearned discounts	$ 45,514	$ 37,261
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, net of unearned discounts	$ 8,529	$ 12,669
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, net of unearned discounts	$ 665	$ 0
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, net of unearned discounts	$ 7,849	$ 2,558
Risk Grade Thirteen [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	13.00	13.00
Loans, net of unearned discounts	$ 20,246	$ 9,671
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, net of unearned discounts	$ 383	$ 2,599
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, net of unearned discounts	$ 0	$ 0