Loans (Activity in Allowance for Loan Losses by Portfolio Segment) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 153,045	$ 135,859	$ 99,542
Provision for loan losses	35,460	51,673	51,845
Charge-offs	(47,804)	(48,328)	(29,572)
Recoveries	14,663	13,841	14,044
Net charge-offs	(33,141)	(34,487)	(15,528)
Ending balance	155,364	153,045	135,859
Individually evaluated for impairment	20,820	9,186	4,378
Collectively evaluated for impairment	134,544	143,859	131,481
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	52,915	42,993	44,273
Provision for loan losses	24,152	22,181	5,255
Charge-offs	(20,619)	(15,910)	(11,092)
Recoveries	3,166	3,651	4,557
Net charge-offs	(17,453)	(12,259)	(6,535)
Ending balance	59,614	52,915	42,993
Individually evaluated for impairment	7,553	5,436	2,378
Collectively evaluated for impairment	52,061	47,479	40,615
Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	60,653	54,696	14,919
Provision for loan losses	884	24,545	45,774
Charge-offs	(10,595)	(18,644)	(6,000)
Recoveries	586	56	3
Net charge-offs	(10,009)	(18,588)	(5,997)
Ending balance	51,528	60,653	54,696
Individually evaluated for impairment	13,267	3,750	2,000
Collectively evaluated for impairment	38,261	56,903	52,696
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	30,213	24,313	27,163
Provision for loan losses	(11)	5,064	(3,182)
Charge-offs	(86)	(82)	(657)
Recoveries	832	918	989
Net charge-offs	746	836	332
Ending balance	30,948	30,213	24,313
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	30,948	30,213	24,313
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	4,238	4,659	5,178
Provision for loan losses	1,925	(164)	(428)
Charge-offs	(925)	(814)	(577)
Recoveries	419	557	486
Net charge-offs	(506)	(257)	(91)
Ending balance	5,657	4,238	4,659
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	5,657	4,238	4,659
Consumer Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	5,026	9,198	8,009
Provision for loan losses	8,510	47	4,426
Charge-offs	(15,579)	(12,878)	(11,246)
Recoveries	9,660	8,659	8,009
Net charge-offs	(5,919)	(4,219)	(3,237)
Ending balance	7,617	5,026	9,198
Individually evaluated for impairment	0	0	0
Collectively evaluated for impairment	$ 7,617	$ 5,026	$ 9,198