Fair Value of Financial Instruments - Corporation's assets measured at fair value at the dates indicated on a nonrecurring basis (Details) - Nonrecurring - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-performing loans	$ 4,532	$ 4,924
Mortgage servicing assets	295	382
Total	4,827	5,306
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-performing loans	0	0
Mortgage servicing assets	0	0
Total	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-performing loans	1,583	2,042
Mortgage servicing assets	0	0
Total	1,583	2,042
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-performing loans	2,949	2,882
Mortgage servicing assets	295	382
Total	$ 3,244	$ 3,264