Recent Developments Including Mergers and Acquisitions - Additional Information (Detail) $ in Thousands	Dec. 14, 2018 USD ($) ParkingSpaces	Aug. 31, 2018 USD ($) ft² Floor	Jan. 11, 2018 USD ($) Community Location	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Combination Separately Recognized Transactions [Line Items]
Goodwill				$ 79,749	$ 79,749
Number of additional banking communities | Community			5
First Bank & Trust Company [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition			Jan. 11, 2018
Total assets acquired			$ 290,000
Loans			247,000
Deposits			251,000
Intangibles assets, net			6,300
Goodwill			$ 19,100
First Bank of Chandler [Member]
Business Combination Separately Recognized Transactions [Line Items]
Date of acquisition			Jan. 11, 2018
Total assets acquired			$ 88,000
Loans			66,000
Deposits			79,000
Intangibles assets, net			2,200
Goodwill			$ 6,600
Number of locations acquired | Location			2
BFTower LLC [Member] | Cotter Ranch Tower [Member]
Business Combination Separately Recognized Transactions [Line Items]
Contract price		$ 21,000
Number of square feet | ft²		507,000
Number of floors | Floor		36
Date of acquisition		Aug. 31, 2018
Percentage of property leased		55.00%
Percentage of occupancy		35.00%
Percentage of occupancy including leased space		90.00%
Renovations completion year		2020
Renovations cost		$ 70,000
BFTower LLC [Member] | Cotter Ranch Tower [Member] | SFPG, LLC [Member]
Business Combination Separately Recognized Transactions [Line Items]
Ownership interest acquired	42.60%
Space of parking garage | ParkingSpaces	1,568
Payment to acquire ownership interest	$ 9,800