Loans and Allowance for Credit Losses - Purchased participation loans, Construction Lending Policy and Construction Loans Modified for other than TDR (Details) $ in Millions	6 Months Ended	12 Months Ended
Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)
Number of purchased participation loans	21	23
Purchased participation loans	$ 66.0	$ 58.2
Construction Loans
Amount of Loans Modified for Other Than TDR	$ 28.5	$ 48.8
Number of Loans Modified for Other Than TDR	51	77

Represents the monetary amount of Amount of Loans Modified for Other Than TDR, as of the indicated date.

No definition available.

Name:	smbc_AmountOfLoansModifiedForOtherThanTdr
Namespace Prefix:	smbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant