Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 149,558	$ 149,714	$ 153,045	$ 135,859
Provision for loan losses	10,980	5,045	27,358	42,734
Charge-offs	(10,354)	(8,516)	(37,253)	(39,194)
Recoveries	4,119	3,530	11,153	10,374
Net charge-offs	(6,235)	(4,986)	(26,100)	(28,820)
Ending balance	154,303	149,773	154,303	149,773
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	48,906	47,578	52,915	42,993
Provision for loan losses	4,096	4,632	7,677	14,315
Charge-offs	(5,468)	(4,036)	(14,574)	(10,754)
Recoveries	903	957	2,419	2,577
Net charge-offs	(4,565)	(3,079)	(12,155)	(8,177)
Ending balance	48,437	49,131	48,437	49,131
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	54,277	66,339	60,653	54,696
Provision for loan losses	(2,815)	(3,231)	1,270	26,170
Charge-offs	0	(884)	(10,595)	(18,644)
Recoveries	451	19	585	21
Net charge-offs	451	(865)	(10,010)	(18,623)
Ending balance	51,913	62,243	51,913	62,243
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	33,002	27,063	30,213	24,313
Provision for loan losses	4,805	1,886	7,086	4,085
Charge-offs	0	(9)	(14)	(56)
Recoveries	268	277	790	875
Net charge-offs	268	268	776	819
Ending balance	38,075	29,217	38,075	29,217
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,535	3,935	4,238	4,659
Provision for loan losses	1,969	427	3,059	(470)
Charge-offs	(766)	(287)	(779)	(464)
Recoveries	137	92	357	442
Net charge-offs	(629)	(195)	(422)	(22)
Ending balance	6,875	4,167	6,875	4,167
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,838	4,799	5,026	9,198
Provision for loan losses	2,925	1,331	8,266	(1,366)
Charge-offs	(4,120)	(3,300)	(11,291)	(9,276)
Recoveries	2,360	2,185	7,002	6,459
Net charge-offs	(1,760)	(1,115)	(4,289)	(2,817)
Ending balance	$ 9,003	$ 5,015	$ 9,003	$ 5,015