Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amounts and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 129,270	$ 135,598
Accumulated Amortization and Impairment	(80,012)	(75,493)
Net Carrying Amount	49,258	60,105
Core deposit
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	112,723	119,051
Accumulated Amortization and Impairment	(66,651)	(63,020)
Net Carrying Amount	46,072	56,031
Customer trust relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	16,547
Accumulated Amortization and Impairment	(13,361)	(12,473)
Net Carrying Amount	$ 3,186	$ 4,074