Organization and Summary of Significant Accounting Policies - Outside Directors' Retirement (Details)	6 Months Ended
Dec. 31, 2020 age
Organization and Summary of Significant Accounting Policies
Requisite period	60
Vesting period	5 years

Minimum period the individual is required to perform services to be fully vested under the retirement plan,

No definition available.

Name:	smbc_RetirementPlanRequisiteServicePeriod
Namespace Prefix:	smbc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration