Available for Sale Securities - Fair Value of Debt Securities Reported Less Than Their Historical Cost (Details) - USD ($) $ in Millions	Jun. 30, 2021	Jun. 30, 2020
Available for Sale Securities
Fair value of certain investments reported less than their historical cost	$ 67.2	$ 10.7
Certain investments in debt securities reported at less than historical cost, percentage of Company's AFS portfolio	32.50%	6.00%

Represents the debt securities reported less than their historical cost percent of investment portfolio, as of the indicated date.

No definition available.

Name:	smbc_DebtSecuritiesReportedLessThanTheirHistoricalCostPercentOfInvestmentPortfolio
Namespace Prefix:	smbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant