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, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	$ 83,038		$ 82,980
Allowance for credit losses	836,674		822,392
Allowance for credit losses	1,008,225	$ 1,001,430	988,997	$ 970,496	$ 962,752	$ 955,992
Purchased Impaired [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	10,396		5,595
Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	930,108		910,967
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	78,117	$ 78,476	78,030	$ 77,722	$ 77,711	$ 78,199
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	50,377		48,480
Allowance for credit losses	288,937		282,353
Commercial, Financial, Leasing, etc. [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	339,314		330,833
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	11,724		12,500
Allowance for credit losses	352,539		348,301
Commercial 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	1,966		1,918
Commercial Real Estate [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	366,229		362,719
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	8,310		9,457
Allowance for credit losses	49,266		47,993
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	8,430		3,677
Residential Real Estate [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	66,006		61,127
Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	12,627		12,543
Allowance for credit losses	145,932		143,745
Consumer Loans [Member] | Allocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for credit losses	$ 158,559		$ 156,288