Loans and Leases and the Allowance for Credit Losses - Loan Modification Activities that were Considered Troubled Debt Restructurings (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Modification	Mar. 31, 2017 USD ($) Modification
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	150	168
Pre-modification recorded investment	$ 70,002	$ 44,692
Post- modification	$ 72,622	$ 39,892
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	20	20
Pre-modification recorded investment	$ 6,780	$ 6,702
Post- modification	6,751	6,597
Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	49,656	13,647
Principal Deferral [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	5,824	2,991
Interest Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	624
Other [Member]
Financing Receivable, Modifications [Line Items]
Post- modification		1,297
Combination of Concession Types [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	22,342	24,948
Combination of Concession Types [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 927	$ 3,606
Commercial, Financial, Leasing, etc. [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	56	50
Pre-modification recorded investment	$ 47,994	$ 11,921
Post- modification	49,344	7,923
Commercial, Financial, Leasing, etc. [Member] | Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	35,673	4,389
Commercial, Financial, Leasing, etc. [Member] | Interest Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	624
Commercial, Financial, Leasing, etc. [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Post- modification		806
Commercial, Financial, Leasing, etc. [Member] | Combination of Concession Types [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 13,047	$ 2,728
Other Commercial Construction [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	1	1
Pre-modification recorded investment	$ 752	$ 102
Post- modification	746	102
Other Commercial Construction [Member] | Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 746	$ 102
Residential [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	47	41
Pre-modification recorded investment	$ 12,636	$ 9,380
Post- modification	13,847	9,948
Residential [Member] | Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	6,945	5,593
Residential [Member] | Combination of Concession Types [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 6,902	$ 4,355
Residential Limited Documentation [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	2	6
Pre-modification recorded investment	$ 295	$ 1,378
Post- modification	385	1,525
Residential Limited Documentation [Member] | Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	267
Residential Limited Documentation [Member] | Combination of Concession Types [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 118	$ 1,525
Home Equity Lines and Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	14	25
Pre-modification recorded investment	$ 1,348	$ 2,502
Post- modification	1,352	2,502
Home Equity Lines and Loans [Member] | Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	4	163
Home Equity Lines and Loans [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Post- modification		491
Home Equity Lines and Loans [Member] | Combination of Concession Types [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 1,348	$ 1,848
Residential Builder and Developer [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification		3
Pre-modification recorded investment		$ 12,291
Post- modification		10,879
Residential Builder and Developer [Member] | Combination of Concession Types [Member]
Financing Receivable, Modifications [Line Items]
Post- modification		$ 10,879
Automobile [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	8	20
Pre-modification recorded investment	$ 148	$ 390
Post- modification	148	390
Automobile [Member] | Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 148	383
Automobile [Member] | Combination of Concession Types [Member]
Financing Receivable, Modifications [Line Items]
Post- modification		$ 7
Other [Member]
Financing Receivable, Modifications [Line Items]
Number of modifications | Modification	2	2
Pre-modification recorded investment	$ 49	$ 26
Post- modification	49	26
Other [Member] | Principal Deferral [Member]
Financing Receivable, Modifications [Line Items]
Post- modification	$ 49	$ 26