Fair Value Measurements and Disclosure - Summary of Fair Value Estimates (Details) - USD ($)	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Financial Assets:
Cash and cash equivalents	$ 0
Investment securities	0		$ 0
Quoted Prices In Active Markets for Identical Assets Level 1
Financial Assets:
Cash and cash equivalents	220,337,000		107,934,000
Investment securities	3,659,000		3,737,000
Loans	0		0
Accrued interest receivable	0		0
Deposits:
Noninterest-bearing	292,720,000		307,299,000
NOW and money market	340,445,000		234,154,000
Savings	90,046,000		81,408,000
Time deposits	0		0
Total deposits	723,211,000		622,861,000
Junior subordinated debt	0		0
Accrued interest payable	0		0
Significant Other Observable Inputs Level 2
Financial Assets:
Cash and cash equivalents			0
Investment securities		$ 66,426,000	41,985,000
Loans	0		0
Accrued interest receivable	8,341,000		6,526,000
Deposits:
Noninterest-bearing	0		0
NOW and money market	0		0
Savings	0		0
Time deposits	0		0
Total deposits	0		0
Junior subordinated debt	0		0
Accrued interest payable	57,000		44,000
Significant Unobservable Inputs Level 3
Financial Assets:
Cash and cash equivalents			0
Investment securities	0		0
Loans	566,195,000		588,938,000
Accrued interest receivable	0		0
Deposits:
Noninterest-bearing	0		0
NOW and money market	0		0
Savings	0		0
Time deposits	81,745,000		64,387,000
Total deposits	81,745,000		64,387,000
Junior subordinated debt	10,155,000		9,730,000
Accrued interest payable	0		0
Carrying Amount
Financial Assets:
Cash and cash equivalents	220,337,000		107,934,000
Investment securities	70,085,000		45,722,000
Loans	579,419,000		593,123,000
Accrued interest receivable	8,341,000		6,526,000
Deposits:
Noninterest-bearing	292,720,000		307,299,000
NOW and money market	340,445,000		234,154,000
Savings	90,046,000		81,408,000
Time deposits	82,432,000		64,832,000
Total deposits	805,643,000		687,693,000
Junior subordinated debt	10,155,000		9,730,000
Accrued interest payable	57,000		44,000
Estimated Fair Value
Financial Assets:
Cash and cash equivalents	220,337,000		107,934,000
Investment securities	70,085,000		45,722,000
Loans	566,195,000		588,938,000
Accrued interest receivable	8,341,000		6,526,000
Deposits:
Noninterest-bearing	292,720,000		307,299,000
NOW and money market	340,445,000		234,154,000
Savings	90,046,000		81,408,000
Time deposits	81,745,000		64,387,000
Total deposits	804,956,000		687,248,000
Junior subordinated debt	10,155,000		9,730,000
Accrued interest payable	$ 57,000		$ 44,000