Significant Accounting Policies - Schedule of Range of Estimated Useful Lives for Premises and Equipment (Detail)	6 Months Ended
	Jun. 30, 2021
Buildings and land improvements [Member] \| Minimum
Property, Plant and Equipment [Line Items]
Property Plant And Equipment	5 years
Buildings and land improvements [Member] \| Maximum
Property, Plant and Equipment [Line Items]
Property Plant And Equipment	50 years
Leasehold Improvements \| Minimum
Property, Plant and Equipment [Line Items]
Property Plant And Equipment	10 years
Leasehold Improvements \| Maximum
Property, Plant and Equipment [Line Items]
Property Plant And Equipment	15 years
Furniture, fixtures and equipment [Member] \| Minimum
Property, Plant and Equipment [Line Items]
Property Plant And Equipment	3 years
Furniture, fixtures and equipment [Member] \| Maximum
Property, Plant and Equipment [Line Items]
Property Plant And Equipment	10 years
Automobiles \| Maximum
Property, Plant and Equipment [Line Items]
Property Plant And Equipment	5 years

CBM BANCORP, INC. 10-Q