Loans and Allowance for Loan Losses - Purchased Credit Impaired Loans Nonaccrual (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jun. 30, 2019
Included in Nonaccrual Loans
Purchased credit impaired loans	$ 0.0	$ 4.1

Represents the monetary amount of Purchased Credit Impaired Loans, as of the indicated date.

No definition available.

Name:	smbc_PurchasedCreditImpairedLoans
Namespace Prefix:	smbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant