Recent Developments Including Mergers and Acquisitions - Additional Information (Detail)						12 Months Ended
	Mar. 05, 2020 USD ($)	Dec. 31, 2019 USD ($)	Aug. 15, 2019 USD ($)	Aug. 31, 2018 USD ($) ft² Floor	Jan. 11, 2018 USD ($) Community Location	Dec. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 14, 2018
Business Combination Separately Recognized Transactions [Line Items]
Goodwill		$ 148,604,000				$ 149,922,000	$ 79,749,000
Number of additional banking communities | Community					5
BFTower LLC [Member] | ParcFirst LLC [Member]
Business Combination Separately Recognized Transactions [Line Items]
Ownership interest acquired		50.00%
Payment to acquire ownership interest		$ 1,500,000
COVID 19 [Member]
Business Combination Separately Recognized Transactions [Line Items]
Loans modification						$ 81,700,000
Citizens Bankshares, Inc [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition	Mar. 05, 2020
Total assets acquired	$ 47,800,000
Loans	22,900,000
Deposits	45,000,000.0
Purchase price	2,900,000
Intangibles assets, net	206,000
Goodwill	$ 1,300,000
Pegasus Bank [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition			Aug. 15, 2019
Total assets acquired			$ 651,100,000
Loans			389,900,000
Deposits			603,900,000
Purchase price			123,500,000
Intangibles assets, net			9,900,000
Goodwill			$ 68,900,000
Cotter Ranch Tower [Member] | BFTower LLC [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition				Aug. 31, 2018
Purchase price				$ 21,000,000.0
Number of square feet | ft²				507,000
Number of floors | Floor				36
Percentage of property leased				55.00%
Percentage of occupancy				35.00%
Percentage of occupancy including leased space				90.00%
Renovations completion year				2022
Renovations cost				$ 70,000,000
Tenant improvements				$ 10,000,000
Number of square feet to be occupied | ft²				165,000
Cotter Ranch Tower [Member] | BFTower LLC [Member] | SFPG, LLC [Member]
Business Combination Separately Recognized Transactions [Line Items]
Ownership interest acquired						49.00%		42.60%
Payment to acquire ownership interest						$ 11,300,000
First Bank & Trust Company [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition					Jan. 11, 2018
Total assets acquired					$ 290,000,000
Loans					247,000,000
Deposits					251,000,000
Intangibles assets, net					6,300,000
Goodwill					$ 19,100,000
First Bank of Chandler [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition					Jan. 11, 2018
Total assets acquired					$ 88,000,000
Loans					66,000,000
Deposits					79,000,000
Intangibles assets, net					2,200,000
Goodwill					$ 6,600,000
Number of locations acquired | Location					2