Goodwill and Other Intangible Assets - Total Amortizing Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 138,421	$ 138,421
Accumulated Amortization	124,256	109,387
Net Carrying Amount	14,165	29,034
Core Deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	131,664	131,664
Accumulated Amortization	119,125	105,802
Net Carrying Amount	12,539	25,862
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	6,757	6,757
Accumulated Amortization	5,131	3,585
Net Carrying Amount	$ 1,626	$ 3,172