Loans (Net (Charge-Offs)/Recoveries, Segregated by Class of Loans) (Detail) $ in Thousands	Dec. 31, 2020 USD ($) Grade	Dec. 31, 2019 USD ($) Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 17,481,309	$ 14,750,332
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.45	6.44
Loans, net of unearned discounts	$ 4,955,341	$ 5,187,466
Commercial Portfolio Segment [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.13	6.17
Loans, net of unearned discounts	$ 4,506,121	$ 4,788,857
Commercial Portfolio Segment [Member] | Risk Grade Nine [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		9.00
Loans, net of unearned discounts	256,198	$ 247,212
Commercial Portfolio Segment [Member] | Risk Grade Ten [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		10.00
Loans, net of unearned discounts	125,977	$ 71,472
Commercial Portfolio Segment [Member] | Risk Grade Eleven [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		11.00
Loans, net of unearned discounts	47,196	$ 53,887
Commercial Portfolio Segment [Member] | Risk Grade Twelve [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		12.00
Loans, net of unearned discounts	14,528	$ 18,189
Commercial Portfolio Segment [Member] | Risk Grade Thirteen [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		13.00
Loans, net of unearned discounts	$ 5,321	$ 7,849
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.85	6.39
Loans, net of unearned discounts	$ 1,235,198	$ 1,652,882
Commercial and Industrial Total Energy Loans [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	5.99	5.90
Loans, net of unearned discounts	$ 968,144	$ 1,488,301
Commercial and Industrial Total Energy Loans [Member] | Risk Grade Nine [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		9.00
Loans, net of unearned discounts	133,547	$ 32,163
Commercial and Industrial Total Energy Loans [Member] | Risk Grade Ten [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		10.00
Loans, net of unearned discounts	46,427	$ 51,898
Commercial and Industrial Total Energy Loans [Member] | Risk Grade Eleven [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		11.00
Loans, net of unearned discounts	63,912	$ 14,760
Commercial and Industrial Total Energy Loans [Member] | Risk Grade Twelve [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		12.00
Loans, net of unearned discounts	13,741	$ 45,514
Commercial and Industrial Total Energy Loans [Member] | Risk Grade Thirteen [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		13.00
Loans, net of unearned discounts	$ 9,427	$ 20,246
Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.34	7.01
Loans, net of unearned discounts	$ 5,796,653	$ 4,883,580
Commercial Real Estate, Buildings, Land and Other [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.97	6.78
Loans, net of unearned discounts	$ 5,047,873	$ 4,523,271
Commercial Real Estate, Buildings, Land and Other [Member] | Risk Grade Nine [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		9.00
Loans, net of unearned discounts	325,227	$ 163,714
Commercial Real Estate, Buildings, Land and Other [Member] | Risk Grade Ten [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		10.00
Loans, net of unearned discounts	258,454	$ 103,626
Commercial Real Estate, Buildings, Land and Other [Member] | Risk Grade Eleven [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		11.00
Loans, net of unearned discounts	149,362	$ 84,057
Commercial Real Estate, Buildings, Land and Other [Member] | Risk Grade Twelve [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		12.00
Loans, net of unearned discounts	15,224	$ 8,529
Commercial Real Estate, Buildings, Land and Other [Member] | Risk Grade Thirteen [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		13.00
Loans, net of unearned discounts	$ 513	$ 383
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.22	7.31
Loans, net of unearned discounts	$ 1,223,814	$ 1,312,659
Construction Loans [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	6.99	7.25
Loans, net of unearned discounts	$ 1,110,025	$ 1,274,098
Construction Loans [Member] | Risk Grade Nine [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		9.00
Loans, net of unearned discounts	72,287	$ 21,509
Construction Loans [Member] | Risk Grade Ten [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		10.00
Loans, net of unearned discounts	38,962	$ 15,243
Construction Loans [Member] | Risk Grade Eleven [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		11.00
Loans, net of unearned discounts	856	$ 1,144
Construction Loans [Member] | Risk Grade Twelve [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		12.00
Loans, net of unearned discounts	1,684	$ 665
Construction Loans [Member] | Risk Grade Thirteen [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		13.00
Loans, net of unearned discounts	$ 0	$ 0
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade	7.32	7.07
Loans, net of unearned discounts	$ 7,020,467	$ 6,196,239
Commercial Real Estate Portfolio Segment [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		6.88
Loans, net of unearned discounts		$ 5,797,369
Commercial Real Estate Portfolio Segment [Member] | Risk Grade Nine [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		9.00
Loans, net of unearned discounts		$ 185,223
Commercial Real Estate Portfolio Segment [Member] | Risk Grade Ten [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		10.00
Loans, net of unearned discounts		$ 118,869
Commercial Real Estate Portfolio Segment [Member] | Risk Grade Eleven [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		11.00
Loans, net of unearned discounts		$ 85,201
Commercial Real Estate Portfolio Segment [Member] | Risk Grade Twelve [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		12.00
Loans, net of unearned discounts		$ 9,194
Commercial Real Estate Portfolio Segment [Member] | Risk Grade Thirteen [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade | Grade		13.00
Loans, net of unearned discounts		$ 383