Fair Value Measurements - Fair Value Measurements, Nonrecurring (Details) - Foreclosed and repossessed assets held for sale - USD ($) $ in Thousands	Jun. 30, 2020	Jun. 30, 2019
Fair value on a nonrecurring basis	$ 2,211	$ 2,430
Fair Value, Inputs, Level 3
Fair value on a nonrecurring basis	$ 2,211	$ 2,430

Represents the monetary amount of Fair value on a nonrecurring basis, as of the indicated date.

No definition available.

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