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COMMITMENTS AND CONTINGENCIES - Service Agreements (Details) - USD ($) $ in Thousands | 6 Months Ended |
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Jun. 30, 2022 | Jun. 18, 2021
Commitments and Contingencies Disclosure [Abstract] |  |
Obligated to pay, year one |  | $ 6,900
Obligated to pay, year two |  | $ 8,000
Payment service provider expenses | $ 7,424 |

X
- Definition Payment of Service Provider Expenses + References No definition available. + Details
Name: | sst_PaymentOfServiceProviderExpenses
Namespace Prefix: | sst_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of contractual obligation to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInNextTwelveMonths
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of contractual obligation to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
Name: | us-gaap_ContractualObligationDueInSecondYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant