Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amounts and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 124,815	$ 129,883
Accumulated Amortization and Impairment	(79,398)	(76,787)
Net Carrying Amount	45,417	53,096	$ 31,876
Core Deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	108,268	108,923
Accumulated Amortization and Impairment	(69,065)	(62,874)
Net Carrying Amount	39,203	46,049
Customer Trust Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	16,547
Accumulated Amortization and Impairment	(10,333)	(9,533)
Net Carrying Amount	$ 6,214	7,014
Customer Loan Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		4,413
Accumulated Amortization and Impairment		(4,380)
Net Carrying Amount		$ 33