Loans Receivable and Other Lending Investments, net (Credit Characteristics by Payment Status) (Details) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan
Recorded investment in loans, aged by payment status and presented by class
Current	$ 933,074	$ 1,062,944
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	61,068	231,877
Total Past Due	61,068	231,877
Loans	$ 994,142	$ 1,294,821
Financing receivable, number of loans greater than 90 days past due \| loan	2	4
Financing receivables, past due time period	90 days	90 days
Minimum
Recorded investment in loans, aged by payment status and presented by class
Financing receivables, past due time period	3 years	1 year
Maximum
Recorded investment in loans, aged by payment status and presented by class
Financing receivables, past due time period	9 years	9 years
Senior mortgages
Recorded investment in loans, aged by payment status and presented by class
Current	$ 792,399	$ 719,057
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	61,068	75,343
Total Past Due	61,068	75,343
Loans	853,467	794,400
Corporate/Partnership loans
Recorded investment in loans, aged by payment status and presented by class
Current	130,823	334,364
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	0	156,534
Total Past Due	0	156,534
Loans	130,823	490,898
Subordinate mortgages
Recorded investment in loans, aged by payment status and presented by class
Current	9,852	9,523
Less Than and Equal to 90 Days	0	0
Greater Than 90 Days	0	0
Total Past Due	0	0
Loans	$ 9,852	$ 9,523

Financing Receivable, Number of Loans Greater than 90 Days Past Due

No definition available.

Name:	star_FinancingReceivableNumberofLoansGreaterthan90DaysPastDue
Namespace Prefix:	star_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Financing receivables that are equal to or less than 90 days past due.

+ References

No definition available.

+ Details

Name:	star_FinancingReceivableRecordedInvestmentEqualToLessThan90DaysPastDue
Namespace Prefix:	star_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing receivables that are greater than 90 days past due.

+ References

No definition available.

+ Details

Name:	star_FinancingReceivableRecordedInvestmentGreaterThan90DaysPastDue
Namespace Prefix:	star_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Financing Receivables, Past Due Time Period

+ References

No definition available.

+ Details

Name:	star_FinancingReceivablesPastDueTimePeriod
Namespace Prefix:	star_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing receivables that are current.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

- Definition

Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_NotesReceivableGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_FirstMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=star_CorporatePartnershipLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_SecondMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: