Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 149,885	$ 153,056	$ 155,364	$ 153,045
Provision for loan losses	8,251	8,426	15,196	16,378
Charge-offs	(11,932)	(15,535)	(28,730)	(26,899)
Recoveries	4,022	3,611	8,396	7,034
Net charge-offs	(7,910)	(11,924)	(20,334)	(19,865)
Ending balance	150,226	149,558	150,226	149,558
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	57,733	45,583	59,614	52,915
Provision for loan losses	3,528	8,184	9,322	3,581
Charge-offs	(4,153)	(5,579)	(13,405)	(9,106)
Recoveries	605	718	2,182	1,516
Net charge-offs	(3,548)	(4,861)	(11,223)	(7,590)
Ending balance	57,713	48,906	57,713	48,906
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	39,039	61,793	51,528	60,653
Provision for loan losses	350	(1,280)	(9,290)	4,085
Charge-offs	(2,689)	(6,317)	(5,539)	(10,595)
Recoveries	613	81	614	134
Net charge-offs	(2,076)	(6,236)	(4,925)	(10,461)
Ending balance	37,313	54,277	37,313	54,277
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	38,474	34,009	30,948	30,213
Provision for loan losses	840	(1,470)	8,283	2,281
Charge-offs	(614)	(14)	(619)	(14)
Recoveries	218	477	306	522
Net charge-offs	(396)	463	(313)	508
Ending balance	38,918	33,002	38,918	33,002
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,349	4,823	5,657	4,238
Provision for loan losses	151	601	1,369	1,090
Charge-offs	(482)	(2)	(1,201)	(13)
Recoveries	318	113	511	220
Net charge-offs	(164)	111	(690)	207
Ending balance	6,336	5,535	6,336	5,535
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,290	6,848	7,617	5,026
Provision for loan losses	3,382	2,391	5,512	5,341
Charge-offs	(3,994)	(3,623)	(7,966)	(7,171)
Recoveries	2,268	2,222	4,783	4,642
Net charge-offs	(1,726)	(1,401)	(3,183)	(2,529)
Ending balance	$ 9,946	$ 7,838	$ 9,946	$ 7,838