Business Combinations - Summary of the Amounts of Assets Acquired and Liabilities Assumed Recognized at the Merger/Acquisition Date (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Oct. 23, 2020	Dec. 31, 2019	Dec. 31, 2018
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill	$ 31,601		$ 136,432	$ 131,712
Almena State Bank [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and due from banks		$ 25,925
Federal Reserve Bank and Federal Home Loan Bank stock		187
Loans		31,407
Premises and equipment		1,109
Other real estate owned		636
Other assets		604
Total assets acquired		66,909
Deposits		62,472
Federal funds purchased and retail repurchase agreements		251
Interest payable and other liabilities		2,041
Total liabilities assumed		64,764
Total identifiable net assets		2,145
Gain on acquisition	$ (2,145)	(2,145)
Almena State Bank [Member] | Available for Sale Securities [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Available-for-sale securities		$ 7,041