Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amounts and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 106,630	$ 129,883
Accumulated Amortization and Impairment	(64,496)	(76,787)
Net Carrying Amount	42,134	53,096	$ 29,235
Core Deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	90,083	108,923
Accumulated Amortization and Impairment	(53,765)	(62,874)
Net Carrying Amount	36,318	46,049
Customer Trust Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	16,547
Accumulated Amortization and Impairment	(10,731)	(9,533)
Net Carrying Amount	$ 5,816	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