Loans (Activity in Allowance for Loan Losses by Portfolio Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	$ 263,177	$ 132,167
Impact of New Accounting Pronouncement	39,377	0	$ 0
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	73,843	51,593	48,580	$ 59,614
Impact of New Accounting Pronouncement	21,263
Financing Receivable, Credit Loss, Expense (Reversal)	15,156	13,144	11,354
Financing Receivable, Allowance for Credit Loss, Writeoff	(18,908)	(14,117)	(26,076)
Financing Receivable, Allowance for Credit Loss, Recovery	4,739	3,986	3,688
Allowance for Loan and Lease Losses Write-offs, Net	(14,169)	(10,131)	(22,388)
Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	39,553	37,382	29,052	51,528
Impact of New Accounting Pronouncement	(10,453)
Financing Receivable, Credit Loss, Expense (Reversal)	85,889	14,388	(9,355)
Financing Receivable, Allowance for Credit Loss, Writeoff	(76,107)	(7,500)	(13,940)
Financing Receivable, Allowance for Credit Loss, Recovery	2,842	1,442	819
Allowance for Loan and Lease Losses Write-offs, Net	(73,265)	(6,058)	(13,121)
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	134,892	31,037	38,777	30,948
Impact of New Accounting Pronouncement	(13,519)
Financing Receivable, Credit Loss, Expense (Reversal)	124,427	(6,934)	8,079
Financing Receivable, Allowance for Credit Loss, Writeoff	(7,499)	(1,025)	(619)
Financing Receivable, Allowance for Credit Loss, Recovery	446	219	369
Allowance for Loan and Lease Losses Write-offs, Net	(7,053)	(806)	(250)
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	7,926	4,113	6,103	5,657
Impact of New Accounting Pronouncement	2,392
Financing Receivable, Credit Loss, Expense (Reversal)	1,906	467	1,984
Financing Receivable, Allowance for Credit Loss, Writeoff	(2,186)	(3,665)	(2,143)
Financing Receivable, Allowance for Credit Loss, Recovery	1,701	1,208	605
Allowance for Loan and Lease Losses Write-offs, Net	(485)	(2,457)	(1,538)
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	6,963	8,042	9,620	7,617
Impact of New Accounting Pronouncement	(2,248)
Financing Receivable, Credit Loss, Expense (Reversal)	9,632	12,694	9,551
Financing Receivable, Allowance for Credit Loss, Writeoff	(17,830)	(24,725)	(17,197)
Financing Receivable, Allowance for Credit Loss, Recovery	9,367	10,453	9,649
Allowance for Loan and Lease Losses Write-offs, Net	(8,463)	(14,272)	(7,548)
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Loss	263,177	132,167	132,132	$ 155,364
Impact of New Accounting Pronouncement	(2,565)
Financing Receivable, Credit Loss, Expense (Reversal)	237,010	33,759	21,613
Financing Receivable, Allowance for Credit Loss, Writeoff	(122,530)	(51,032)	(59,975)
Financing Receivable, Allowance for Credit Loss, Recovery	19,095	17,308	15,130
Allowance for Loan and Lease Losses Write-offs, Net	$ (103,435)	$ (33,724)	$ (44,845)