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Real Estate (Real Estate Available and Held for Sale) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Real Estate Properties [Line Items] |  |  |
Property transferred to held for sale, aggregate, carrying value | $ 200.4 | $ 31.3 | $ 21.0
Operating Properties |  |  |
Real Estate Properties [Line Items] |  |  |
Property transferred to held for sale, aggregate, carrying value | 14.5 | 23.2 | 20.1
Net Lease |  |  |
Real Estate Properties [Line Items] |  |  |
Property transferred to held for sale, aggregate, carrying value | $ 185.9 | $ 8.1 | $ 0.9

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- Definition Property Transferred to Held for Sale, Aggregate, Carrying Value + References No definition available. + Details
Name: | star_PropertyTransferredtoHeldforSaleAggregateCarryingValue
Namespace Prefix: | star_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_RealEstatePropertiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_RealEstatePropertiesAxis=star_OperatingPropertiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RealEstatePropertiesAxis=star_NetLeaseAssetMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |