Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 155,364	$ 153,045
Provision for loan losses	6,945	7,952
Charge-offs	(16,798)	(11,364)
Recoveries	4,374	3,423
Net charge-offs	(12,424)	(7,941)
Ending balance	149,885	153,056
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	59,614	52,915
Provision for loan losses	5,794	(4,603)
Charge-offs	(9,252)	(3,527)
Recoveries	1,577	798
Net charge-offs	(7,675)	(2,729)
Ending balance	57,733	45,583
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	51,528	60,653
Provision for loan losses	(9,640)	5,365
Charge-offs	(2,850)	(4,278)
Recoveries	1	53
Net charge-offs	(2,849)	(4,225)
Ending balance	39,039	61,793
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	30,948	30,213
Provision for loan losses	7,443	3,751
Charge-offs	(5)	0
Recoveries	88	45
Net charge-offs	83	45
Ending balance	38,474	34,009
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,657	4,238
Provision for loan losses	1,218	489
Charge-offs	(719)	(11)
Recoveries	193	107
Net charge-offs	(526)	96
Ending balance	6,349	4,823
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,617	5,026
Provision for loan losses	2,130	2,950
Charge-offs	(3,972)	(3,548)
Recoveries	2,515	2,420
Net charge-offs	(1,457)	(1,128)
Ending balance	$ 8,290	$ 6,848