Debt Instrument, Periodic Principal Payment, Percent

No definition available.

Name:	star_DebtInstrumentPeriodicPrincipalPaymentPercent
Namespace Prefix:	star_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of principal amount used in the computation of issue price for the line of credit facility.

+ References

No definition available.

+ Details

Name:	star_LineOfCreditFacilityIssuePriceAsPercentageOfPrincipalAmount
Namespace Prefix:	star_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=114001942&loc=d3e12355-112629

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114001942&loc=d3e12317-112629

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=star_A2016SeniorSecuredCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=star_SecuredCreditFacilityRevolverDueMarch2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=star_SecuredRevolvingCreditFacilityDueSeptember2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt Obligations, net - Secured Term Loan and Credit Facility Narrative (Details) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Aug. 31, 2016	Jun. 30, 2016	Mar. 31, 2015
Line of Credit Facility [Line Items]
Loss on early extinguishment of debt			$ 0	$ 2,164,000	$ 468,000	$ 2,536,000
2016 Senior Term Loan \| LIBOR
Line of Credit Facility [Line Items]
Basis point spread on variable interest rate (as a percent)					1.00%
Secured Debt \| 2016 Senior Term Loan
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 650,000,000.0	$ 400,000,000.0		650,000,000.0		650,000,000.0	$ 500,000,000.0	$ 450,000,000.0
Percentage of par credit facilities were issued at								99.00%
Required quarterly principal payment (as a percent)					0.25%
Loss on early extinguishment of debt				$ 2,200,000		$ 2,500,000
Secured Debt \| 2016 Senior Term Loan \| LIBOR
Line of Credit Facility [Line Items]
Basis point spread on variable interest rate (as a percent)	2.75%	3.00%			2.75%
Secured Debt \| 2016 Senior Term Loan \| LIBOR \| Minimum
Line of Credit Facility [Line Items]
Basis point spread on variable interest rate (as a percent)		0.75%
Secured Debt \| 2016 Senior Term Loan \| LIBOR \| Maximum
Line of Credit Facility [Line Items]
Basis point spread on variable interest rate (as a percent)					1.75%
Secured Debt \| 2015 Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity									$ 250,000,000.0
Secured Debt \| 2015 $325 million Revolving Credit Facility
Line of Credit Facility [Line Items]
Debt instrument, face amount			325,000,000		$ 325,000,000
Maximum borrowing capacity		$ 325,000,000.0	325,000,000.0		$ 325,000,000.0
Debt instrument term (in years)					1 year
Current borrowing capacity			$ 325,000,000.0		$ 325,000,000.0
Secured Debt \| 2015 $325 million Revolving Credit Facility \| Minimum
Line of Credit Facility [Line Items]
Commitment fee percentage					0.30%
Secured Debt \| 2015 $325 million Revolving Credit Facility \| Maximum
Line of Credit Facility [Line Items]
Commitment fee percentage					0.50%
Secured Debt \| 2015 $325 million Revolving Credit Facility \| LIBOR
Line of Credit Facility [Line Items]
Basis point spread on variable interest rate (as a percent)					2.50%
Secured Debt \| 2015 $325 million Revolving Credit Facility \| LIBOR \| Minimum
Line of Credit Facility [Line Items]
Basis point spread on variable interest rate (as a percent)					1.00%