Fair Value Measurements - Unobservable (Level 3) inputs (Details) - Nonrecurring Measurements - Fair Value, Inputs, Level 3 - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2023	Jun. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements Nonrecurring Unobservable Inputs	$ 1,609	$ 1,472
Fair Value Measurements Nonrecurring Weighted Average Discount Applied	1.1	14.9
Investment, Type [Extensible Enumeration]	smbc:ForeclosedAndRepossessedAssetsMember	smbc:ForeclosedAndRepossessedAssetsMember
Third party appraisal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements Nonrecurring Valuation Technique	Third party appraisal	Third party appraisal
Third party appraisal | Marketability discount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements Nonrecurring Unobservable Inputs	Marketability discount	Marketability discount
Third party appraisal | Marketability discount | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements Nonrecurring Range of discounts Applied	0.00%	14.90%
Third party appraisal | Marketability discount | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements Nonrecurring Range of discounts Applied	5.60%	14.90%