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Other Investments (Narrative) (Details) |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 27, 2017 USD ($) | Apr. 14, 2017 USD ($) | Jun. 30, 2018 USD ($) lease facility | May 31, 2018 USD ($) | Oct. 31, 2017 USD ($) unit | Aug. 31, 2017 USD ($) | Apr. 30, 2017 USD ($) property | Dec. 31, 2016 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) plan $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Jan. 01, 2018 USD ($) | Mar. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on consolidation of equity method investment |  |  |  |  |  |  |  |  | $ 67,877,000 |  | $ 0 |  | $ 67,877,000 |  | $ 0 |  |  |  |  |
Increase in noncontrolling interest from consolidation of equity method investment |  |  |  |  |  |  |  |  |  |  |  |  | 188,279,000 |  |  |  |  |  |  |
Increase in redeemable noncontrolling interest from consolidation of equity method investment |  |  |  |  |  |  |  |  |  |  |  |  | 11,800,000 |  |  |  |  |  |  |
Payments to acquire equity method investments |  |  |  |  |  |  |  |  |  |  |  |  | 53,012,000 |  | 139,139,000 |  |  |  |  |
Carrying value of assets |  |  | $ 5,358,167,000 |  |  |  |  |  | 5,358,167,000 |  |  |  | 5,358,167,000 |  |  |  | $ 5,358,167,000 | $ 4,731,078,000 |  |
Net proceeds from sales of real estate |  |  | 36,100,000 |  |  |  |  |  |  |  |  |  | 238,834,000 |  | 154,291,000 |  |  |  |  |
Gain from discontinued operations |  |  |  |  |  |  |  |  | 0 |  | 123,418,000 |  | 0 |  | 123,418,000 |  |  |  |  |
Retained deficit |  |  | (2,325,291,000) |  |  |  |  |  | (2,325,291,000) |  |  |  | (2,325,291,000) |  |  |  | (2,325,291,000) | (2,470,564,000) |  |
Debt obligation |  |  | 3,928,372,000 |  |  |  |  |  | 3,928,372,000 |  |  |  | 3,928,372,000 |  |  |  | 3,928,372,000 | 3,539,991,000 |  |
Carrying value of loans |  |  | $ 989,002,000 |  |  |  |  |  | 989,002,000 |  |  |  | 989,002,000 |  |  |  | 989,002,000 | 1,289,568,000 |  |
Number of industrial facilities sold | facility |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of ground leases structured and entered into | lease |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of ground leases sold | lease |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale of real estate |  |  | $ 24,500,000 |  |  |  |  |  | 56,895,000 | [1] | 844,000 | [1] | 73,943,000 | [1] | 8,954,000 | [1] |  |  |  |
Equity method investments, carrying value |  |  | $ 293,017,000 |  |  |  |  |  | $ 293,017,000 |  |  |  | $ 293,017,000 |  |  |  | $ 293,017,000 | 321,241,000 |  |
iStar Net Lease II LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest in lease acquisition and development venture (percent) |  |  | 51.90% |  |  |  |  |  | 51.90% |  |  |  | 51.90% |  |  |  | 51.90% |  |  |
Net lease venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, related party ownership percentage |  |  | 0.60% |  |  |  |  |  | 0.60% |  |  |  | 0.60% |  |  |  | 0.60% |  |  |
Equity method investment, related party promote fee percentage |  |  | 50.00% |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  | 50.00% |  |  |
Equity method investment, partner ownership percentage |  |  | 47.50% |  |  |  |  |  | 47.50% |  |  |  | 47.50% |  |  |  | 47.50% |  |  |
Net lease venture | Other income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees revenue |  |  |  |  |  |  |  |  | $ 700,000 |  | 500,000 |  | $ 1,300,000 |  | 900,000 |  |  |  |  |
Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees waived |  |  |  |  |  |  |  |  | 900,000 |  |  |  | 1,800,000 |  |  |  |  |  |  |
Reimbursement revenue waived |  |  |  |  |  |  |  |  | 400,000 |  |  |  | 800,000 |  |  |  |  |  |  |
Other real estate equity investments | Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  |  |  |  |  |  |  | $ 7,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments, carrying value |  |  | $ 62,000,000 |  |  |  |  |  | 62,000,000 |  |  |  | 62,000,000 |  |  |  | $ 62,000,000 | 38,800,000 |  |
Loan commitments |  |  |  |  |  |  |  | $ 27,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Loans receivable, carrying value |  |  | 26,800,000 |  |  |  |  |  | 26,800,000 |  |  |  | 26,800,000 |  |  |  | 26,800,000 | 25,400,000 |  |
Interest income |  |  |  |  |  |  |  |  | 500,000 |  | 500,000 |  | 1,000,000 |  | 900,000 |  |  |  |  |
Other real estate equity investments | Land & Development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments, carrying value |  |  | $ 76,700,000 |  |  |  |  |  | $ 76,700,000 |  |  |  | $ 76,700,000 |  |  |  | $ 76,700,000 | $ 61,300,000 |  |
Other real estate equity investments | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest in lease acquisition and development venture (percent) |  |  | 15.50% |  |  |  |  |  | 15.50% |  |  |  | 15.50% |  |  |  | 15.50% |  |  |
Other real estate equity investments | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest in lease acquisition and development venture (percent) |  |  | 95.00% |  |  |  |  |  | 95.00% |  |  |  | 95.00% |  |  |  | 95.00% |  |  |
Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lessor, operating lease, term of contract |  |  |  |  | 99 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real estate, leasehold improvement allowance |  |  |  |  | $ 7,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward purchase contract, purchase agreement, amount |  |  |  |  | $ 34,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units in real estate property to be built | unit |  |  |  |  | 301 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Great Oaks | Leasehold First Mortgage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt obligation |  |  |  |  | $ 80,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of loans |  |  |  |  | $ 80,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Newly formed unconsolidated entity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest in lease acquisition and development venture (percent) |  |  | 50.00% |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  | 50.00% |  |  |
Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest in lease acquisition and development venture (percent) |  |  | 39.80% |  |  |  |  |  | 39.80% |  |  |  | 39.80% |  |  |  | 39.80% |  |  |
Contributions by majority shareholders |  | $ 57,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of assets |  | 161,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain from discontinued operations |  |  |  |  |  |  | $ 123,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale of real estate |  |  |  |  |  |  | 123,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Ground Lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  | $ 19,900,000 |  | $ 24,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan term |  |  |  | 1 year |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loan funded |  |  | $ 13,000,000 |  |  |  |  |  | $ 13,000,000 |  |  |  | $ 13,000,000 |  |  |  | $ 13,000,000 |  |  |
Safety, Income and Growth, Inc. | Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance initial public offering | $ 205,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of private placement | 45,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | IPO |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments of stock issuance costs | 18,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Offering costs | $ 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Wholly-owned Subsidiary | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee, percent of equity below threshold |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |  |  |
Management fee, shareholders' equity threshold amount |  |  | $ 2,500,000,000.0 |  |  |  |  |  | $ 2,500,000,000.0 |  |  |  | $ 2,500,000,000.0 |  |  |  | $ 2,500,000,000.0 |  |  |
Management fee, percent of equity above threshold |  |  |  |  |  |  |  |  |  |  |  |  | 0.75% |  |  |  |  |  |  |
Ground Net Lease Business | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  | $ 55,500,000 |  |  |  |  | 55,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest (as a percent) |  | 49.10% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 50.90% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from sales of real estate |  | $ 113,000,000 |  |  |  |  | $ 113,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties | property |  |  |  |  |  |  | 12 |  |  |  |  |  |  |  |  |  |  |  |  |
2017 Secured Financing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  |  | $ 227,000,000.0 |  |  |  | $ 227,000,000.0 |  |  |  |  | $ 227,000,000.0
10b5-1 Plan | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares purchased | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,200,000 |  |  |
Aggregate value of common stock purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 41,700,000 |  |  |
Average share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 18.67 |  |  |
Common stock purchase plan, number of plans | plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
Chief Executive Officer and Chief Financial Officer | 10b5-1 Plan | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares purchased | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 26,000 |  |
Aggregate value of common stock purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000 |  |
Average share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 19.20 |  |
Accounting Standards Update 2017-05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,806,947,000 |
Retained deficit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (2,394,695,000) |
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2017-05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Retained deficit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 55,500,000 |
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2017-05 | Ground Net Lease Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale of business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 178,900,000 |  |
[1] | Income from sales of real estate represents gains from sales of real estate that do not qualify as discontinued operations

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