Fair Value Measurements - Losses recognized on assets measured on a non-recurring basis (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Mar. 31, 2020	Mar. 31, 2021
Foreclosed and repossessed assets held for sale
Gains (losses) on assets measured on a non-recurring basis	$ (96)	$ (49)
Total losses on assets measured on a non-recurring basis
Gains (losses) on assets measured on a non-recurring basis	$ (96)	$ (49)

Represents the monetary amount of Gains (losses) recognized on assets measured on a non-recurring basis, during the indicated time period.

No definition available.

Name:	smbc_GainsLossesRecognizedOnAssetsMeasuredOnANonRecurringBasis
Namespace Prefix:	smbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration