Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amounts and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 135,598	$ 135,598
Accumulated Amortization and Impairment	(79,269)	(75,493)
Net Carrying Amount	56,329	60,105
Core deposit
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	119,051	119,051
Accumulated Amortization and Impairment	(66,488)	(63,020)
Net Carrying Amount	52,563	56,031
Customer trust relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	16,547
Accumulated Amortization and Impairment	(12,781)	(12,473)
Net Carrying Amount	$ 3,766	$ 4,074