Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amounts and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 135,598	$ 145,647
Accumulated Amortization and Impairment	(71,547)	(68,631)
Net Carrying Amount	64,051	77,016	$ 42,134
Core Deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	119,051	129,100
Accumulated Amortization and Impairment	(59,398)	(57,524)
Net Carrying Amount	59,653	71,576
Customer Trust Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	16,547
Accumulated Amortization and Impairment	(12,149)	(11,107)
Net Carrying Amount	$ 4,398	$ 5,440