Loans and Leases and the Allowance for Credit Losses - Allocation of Allowance for Credit Losses on Basis of Company's Impairment Methodology (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	$ 67,677		$ 79,646
Allowance for credit losses	877,180		849,356
Allowance for credit losses	1,029,867	$ 1,019,337	1,019,444	$ 1,019,248	$ 1,019,671	$ 1,017,198
Purchased Impaired [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	6,525		12,397
Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	951,382		941,399
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	78,485	$ 77,541	78,045	$ 77,547	$ 78,995	$ 78,300
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	36,496		46,034
Allowance for credit losses	299,359		284,021
Commercial, Financial, Leasing, etc. [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	335,855		330,055
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	10,463		13,039
Allowance for credit losses	332,833		328,616
Commercial Real Estate [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	343,296		341,655
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	8,820		8,402
Allowance for credit losses	45,666		48,326
Residential Real Estate [Member] | Purchased Impaired [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	6,525		12,397
Residential Real Estate [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	61,011		69,125
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	11,898		12,171
Allowance for credit losses	199,322		188,393
Consumer Loans [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	$ 211,220		$ 200,564