Fair Value Measurements - Gains (Losses) Recognized on Assets Measured on a Nonrecurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Foreclosed and repossessed assets held for sale
Gains (losses) recognized on assets measured on a non-recurring basis	$ (1,009)	$ (353)
Total losses on assets measured on a non-recurring basis
Gains (losses) recognized on assets measured on a non-recurring basis	$ (1,009)	$ (353)

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