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	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	$ 76,409	$ 75,762
Allowance for credit losses	850,361	853,276
Allowance for credit losses	1,019,671	1,017,198	$ 1,001,430	$ 988,997
Purchased Impaired [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	13,906	9,860
Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	940,676	938,898
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	78,995	78,300	$ 78,476	$ 78,030
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	45,501	45,488
Allowance for credit losses	280,570	283,111
Commercial, Financial, Leasing, etc. [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	326,071	328,599
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	10,624	10,095
Allowance for credit losses	357,093	363,990
Commercial Real Estate [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	367,717	374,085
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	8,048	7,900
Allowance for credit losses	51,093	47,645
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	13,906	9,860
Residential Real Estate [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	73,047	65,405
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	12,236	12,279
Allowance for credit losses	161,605	158,530
Consumer Loans [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	$ 173,841	$ 170,809