Loans (Activity in Allowance for Loan Losses by Portfolio Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 132,132	$ 155,364	$ 153,045
Provision for loan losses	33,759	21,613	35,460
Charge-offs	(51,032)	(59,975)	(47,804)
Recoveries	17,308	15,130	14,663
Net charge-offs	(33,724)	(44,845)	(33,141)
Ending balance	132,167	132,132	155,364
Individually evaluated for impairment	28,483	16,235	20,820
Collectively evaluated for impairment	103,684	115,897	134,544
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	48,580	59,614	52,915
Provision for loan losses	13,144	11,354	24,152
Charge-offs	(14,117)	(26,076)	(20,619)
Recoveries	3,986	3,688	3,166
Net charge-offs	(10,131)	(22,388)	(17,453)
Ending balance	51,593	48,580	59,614
Individually evaluated for impairment	7,849	2,558	7,553
Collectively evaluated for impairment	43,744	46,022	52,061
Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	29,052	51,528	60,653
Provision for loan losses	14,388	(9,355)	884
Charge-offs	(7,500)	(13,940)	(10,595)
Recoveries	1,442	819	586
Net charge-offs	(6,058)	(13,121)	(10,009)
Ending balance	37,382	29,052	51,528
Individually evaluated for impairment	20,246	9,671	13,267
Collectively evaluated for impairment	17,136	19,381	38,261
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	38,777	30,948	30,213
Provision for loan losses	(6,934)	8,079	(11)
Charge-offs	(1,025)	(619)	(86)
Recoveries	219	369	832
Net charge-offs	(806)	(250)	746
Ending balance	31,037	38,777	30,948
Individually evaluated for impairment	383	2,599	0
Collectively evaluated for impairment	30,654	36,178	30,948
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	6,103	5,657	4,238
Provision for loan losses	467	1,984	1,925
Charge-offs	(3,665)	(2,143)	(925)
Recoveries	1,208	605	419
Net charge-offs	(2,457)	(1,538)	(506)
Ending balance	4,113	6,103	5,657
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	4,113	6,103	5,657
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	9,620	7,617	5,026
Provision for loan losses	12,694	9,551	8,510
Charge-offs	(24,725)	(17,197)	(15,579)
Recoveries	10,453	9,649	9,660
Net charge-offs	(14,272)	(7,548)	(5,919)
Ending balance	8,042	9,620	7,617
Individually evaluated for impairment	5	1,407	0
Collectively evaluated for impairment	$ 8,037	$ 8,213	$ 7,617