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Stock-Based Compensation Plans and Employee Benefits (401(k) Plan) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |  |  |
Requisite service period | 3 months |  |
Defined contribution plan maximum percentage of matching contribution | 50.00% |  |
Defined contribution plan maximum percentage of employee compensation | 10.00% |  |
Gross contributions made by the Company | $ 1.1 | $ 1.1 | $ 1.0

X
- Definition At the discretion of the Board of Directors, represents the maximum percentage matching contribution which the Company may make of the first ten percent of the participant's annual compensation. + References No definition available. + Details
Name: | star_DefinedContributionPlanMaximumPercentageOfMatchingContributionCompanyMayMakeOfFirstTenPercentageOfParticipatingEmployeesAnnualCompensationAtDiscretionOfBoardOfDirectors
Namespace Prefix: | star_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the period of continuous service with the Company required in order to be eligible to participate in the 401(k) Plan. + References No definition available. + Details
Name: | star_DefinedContributionPlanServiceRequirement
Namespace Prefix: | star_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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