Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amounts and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 145,647	$ 129,883
Accumulated Amortization and Impairment	(68,631)	(76,787)
Net Carrying Amount	77,016	53,096
Core Deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	129,100	108,923
Accumulated Amortization and Impairment	(57,524)	(62,874)
Net Carrying Amount	71,576	46,049
Customer Trust Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	16,547
Accumulated Amortization and Impairment	(11,107)	(9,533)
Net Carrying Amount	$ 5,440	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