Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 132,132	$ 155,364
Provision for loan losses	11,003	6,945
Charge-offs	(10,223)	(16,798)
Recoveries	3,438	4,374
Net charge-offs	(6,785)	(12,424)
Ending balance	136,350	149,885
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	48,580	59,614
Provision for loan losses	11,929	5,794
Charge-offs	(2,688)	(9,252)
Recoveries	750	1,577
Net charge-offs	(1,938)	(7,675)
Ending balance	58,571	57,733
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	29,052	51,528
Provision for loan losses	(3,756)	(9,640)
Charge-offs	0	(2,850)
Recoveries	47	1
Net charge-offs	47	(2,849)
Ending balance	25,343	39,039
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	38,777	30,948
Provision for loan losses	(2,352)	7,443
Charge-offs	(60)	(5)
Recoveries	90	88
Net charge-offs	30	83
Ending balance	36,455	38,474
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,103	5,657
Provision for loan losses	1,247	1,218
Charge-offs	(1,778)	(719)
Recoveries	89	193
Net charge-offs	(1,689)	(526)
Ending balance	5,661	6,349
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	9,620	7,617
Provision for loan losses	3,935	2,130
Charge-offs	(5,697)	(3,972)
Recoveries	2,462	2,515
Net charge-offs	(3,235)	(1,457)
Ending balance	$ 10,320	$ 8,290