Allowance for Credit Losses - Changes in Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 988,997	$ 955,992	$ 919,562
Provision for credit losses	168,000	190,000	170,000
Net charge-offs
Charge-offs	(224,598)	(231,255)	(198,373)
Recoveries	84,799	74,260	64,803
Net (charge-offs) recoveries	(139,799)	(156,995)	(133,570)
Ending balance	1,017,198	988,997	955,992
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	330,833	300,404	288,038
Provision for credit losses	41,511	59,506	43,065
Net charge-offs
Charge-offs	(64,941)	(59,244)	(60,983)
Recoveries	21,196	30,167	30,284
Net (charge-offs) recoveries	(43,745)	(29,077)	(30,699)
Ending balance	328,599	330,833	300,404
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	362,719	326,831	307,927
Provision for credit losses	6,715	33,627	25,768
Net charge-offs
Charge-offs	(7,931)	(4,805)	(16,487)
Recoveries	12,582	7,066	9,623
Net (charge-offs) recoveries	4,651	2,261	(6,864)
Ending balance	374,085	362,719	326,831
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	61,127	72,238	61,910
Provision for credit losses	16,094	6,902	19,133
Net charge-offs
Charge-offs	(20,799)	(26,133)	(13,116)
Recoveries	8,983	8,120	4,311
Net (charge-offs) recoveries	(11,816)	(18,013)	(8,805)
Ending balance	65,405	61,127	72,238
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	156,288	178,320	186,033
Provision for credit losses	103,410	90,134	79,489
Net charge-offs
Charge-offs	(130,927)	(141,073)	(107,787)
Recoveries	42,038	28,907	20,585
Net (charge-offs) recoveries	(88,889)	(112,166)	(87,202)
Ending balance	170,809	156,288	178,320
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	78,030	78,199	75,654
Provision for credit losses	270	(169)	2,545
Net charge-offs
Ending balance	$ 78,300	$ 78,030	$ 78,199