Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 150,226	$ 149,558	$ 155,364	$ 153,045
Provision for loan losses	2,650	10,980	17,846	27,358
Charge-offs	(18,596)	(10,354)	(47,326)	(37,253)
Recoveries	3,298	4,119	11,694	11,153
Net charge-offs	(15,298)	(6,235)	(35,632)	(26,100)
Ending balance	137,578	154,303	137,578	154,303
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	57,713	48,906	59,614	52,915
Provision for loan losses	1,085	4,096	10,407	7,677
Charge-offs	(8,491)	(5,468)	(21,896)	(14,574)
Recoveries	684	903	2,866	2,419
Net charge-offs	(7,807)	(4,565)	(19,030)	(12,155)
Ending balance	50,991	48,437	50,991	48,437
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	37,313	54,277	51,528	60,653
Provision for loan losses	(521)	(2,815)	(9,811)	1,270
Charge-offs	(5,400)	0	(10,939)	(10,595)
Recoveries	53	451	667	585
Net charge-offs	(5,347)	451	(10,272)	(10,010)
Ending balance	31,445	51,913	31,445	51,913
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	38,918	33,002	30,948	30,213
Provision for loan losses	250	4,805	8,533	7,086
Charge-offs	0	0	(619)	(14)
Recoveries	36	268	342	790
Net charge-offs	36	268	(277)	776
Ending balance	39,204	38,075	39,204	38,075
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,336	5,535	5,657	4,238
Provision for loan losses	246	1,969	1,615	3,059
Charge-offs	(431)	(766)	(1,632)	(779)
Recoveries	43	137	554	357
Net charge-offs	(388)	(629)	(1,078)	(422)
Ending balance	6,194	6,875	6,194	6,875
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,946	7,838	7,617	5,026
Provision for loan losses	1,590	2,925	7,102	8,266
Charge-offs	(4,274)	(4,120)	(12,240)	(11,291)
Recoveries	2,482	2,360	7,265	7,002
Net charge-offs	(1,792)	(1,760)	(4,975)	(4,289)
Ending balance	$ 9,744	$ 9,003	$ 9,744	$ 9,003