Loans - Activity in Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 153,045	$ 135,859
Provision for loan losses	7,952	28,500
Charge-offs	(11,364)	(5,778)
Recoveries	3,423	3,299
Net charge-offs	(7,941)	(2,479)
Ending balance	153,056	161,880
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	52,915	42,993
Provision for loan losses	(4,603)	3,223
Charge-offs	(3,527)	(1,861)
Recoveries	798	729
Net charge-offs	(2,729)	(1,132)
Ending balance	45,583	45,084
Commercial and industrial, Energy [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	60,653	54,696
Provision for loan losses	5,365	31,288
Charge-offs	(4,278)	(1,011)
Recoveries	53	0
Net charge-offs	(4,225)	(1,011)
Ending balance	61,793	84,973
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	30,213	24,313
Provision for loan losses	3,751	(794)
Charge-offs	0	(28)
Recoveries	45	96
Net charge-offs	45	68
Ending balance	34,009	23,587
Consumer Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	4,238	4,659
Provision for loan losses	489	(972)
Charge-offs	(11)	(154)
Recoveries	107	253
Net charge-offs	96	99
Ending balance	4,823	3,786
Consumer and Other [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,026	9,198
Provision for loan losses	2,950	(4,245)
Charge-offs	(3,548)	(2,724)
Recoveries	2,420	2,221
Net charge-offs	(1,128)	(503)
Ending balance	$ 6,848	$ 4,450