Loans - Activity in the Allowance for Credit Losses on Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance			$ 132,167
Impact of adopting ASC 326	$ 0	$ 0	39,377	$ 0
Ending balance	250,061		250,061
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	92,152	58,571	51,593	48,580
Impact of adopting ASC 326			21,263
Credit loss expense	7,334	1,597	29,284	13,526
Charge-offs	(1,841)	(3,389)	(6,210)	(6,077)
Recoveries	891	935	2,606	1,685
Net charge-offs	(950)	(2,454)	(3,604)	(4,392)
Ending balance	98,536	57,714	98,536	57,714
Commercial and Industrial Total Energy Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	103,201	25,343	37,382	29,052
Impact of adopting ASC 326			(10,453)
Credit loss expense	(27,342)	2,446	82,664	(1,310)
Charge-offs	(35,042)	(2,000)	(68,842)	(2,000)
Recoveries	0	29	66	76
Net charge-offs	(35,042)	(1,971)	(68,776)	(1,924)
Ending balance	40,817	25,818	40,817	25,818
Commercial Real Estate Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	52,319	36,455	31,037	38,777
Impact of adopting ASC 326			(13,519)
Credit loss expense	44,587	(13)	79,348	(2,365)
Charge-offs	(3,511)	(557)	(3,584)	(617)
Recoveries	30	29	143	119
Net charge-offs	(3,481)	(528)	(3,441)	(498)
Ending balance	93,425	35,914	93,425	35,914
Total consumer real estate loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,170	5,661	4,113	6,103
Impact of adopting ASC 326			2,392
Credit loss expense	352	262	1,922	1,509
Charge-offs	(135)	(601)	(420)	(2,379)
Recoveries	611	315	991	404
Net charge-offs	476	(286)	571	(1,975)
Ending balance	8,998	5,637	8,998	5,637
Consumer Loan [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8,039	10,320	8,042	9,620
Impact of adopting ASC 326			(2,248)
Credit loss expense	2,297	2,108	6,935	6,043
Charge-offs	(4,178)	(5,103)	(9,183)	(10,800)
Recoveries	2,127	2,521	4,739	4,983
Net charge-offs	(2,051)	(2,582)	(4,444)	(5,817)
Ending balance	8,285	9,846	8,285	9,846
Loans Receivable [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	263,881	136,350	132,167	132,132
Impact of adopting ASC 326			(2,565)
Credit loss expense	27,228	6,400	200,153	17,403
Charge-offs	(44,707)	(11,650)	(88,239)	(21,873)
Recoveries	3,659	3,829	8,545	7,267
Net charge-offs	(41,048)	(7,821)	(79,694)	(14,606)
Ending balance	$ 250,061	$ 134,929	$ 250,061	$ 134,929