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Real Estate - Tenant Reimbursements (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Real Estate [Abstract] |  |  |  |
Tenant expense reimbursements | $ 5.8 | $ 4.9 | $ 17.1 | $ 14.8

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- Definition Tenant Expense Reimbursements + References No definition available. + Details
Name: | star_TenantExpenseReimbursements
Namespace Prefix: | star_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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