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Other Investments (Narrative) (Details) |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 27, 2017 USD ($) | Apr. 14, 2017 USD ($) investor | Nov. 01, 2018 shares | Jun. 30, 2018 USD ($) lease facility | May 31, 2018 USD ($) office_building | Oct. 31, 2017 USD ($) unit | Aug. 31, 2017 USD ($) | Apr. 30, 2017 USD ($) property | Sep. 30, 2018 USD ($) $ / shares shares | Sep. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) plan $ / shares shares | Dec. 31, 2017 USD ($) | Aug. 31, 2018 USD ($) | Jan. 01, 2018 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on consolidation of equity method investment |  |  |  |  |  |  |  |  | $ 0 |  | $ 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 |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 122,141,000 |  | 119,872,000 |  |  | 657,956,000 |  | 576,059,000 |  |  |  |  |  |  |
Payments to acquire equity method investments |  |  |  |  |  |  |  |  |  |  |  |  |  | 68,666,000 |  | 181,279,000 |  |  |  |  |  |  |
Carrying value of assets |  |  |  |  |  |  |  |  | 5,079,989,000 |  |  |  |  | 5,079,989,000 |  |  |  |  | $ 4,731,078,000 |  |  |  |
Net proceeds from sales of real estate |  |  |  | $ 36,100,000 |  |  |  |  |  |  |  |  |  | 271,358,000 |  | 201,939,000 |  |  |  |  |  |  |
Gain from discontinued operations |  |  |  |  |  |  |  |  | 0 |  | 0 |  |  | 0 |  | 123,418,000 |  |  |  |  |  |  |
Carrying value of loans |  |  |  |  |  |  |  |  | 963,975,000 |  |  |  |  | 963,975,000 |  |  |  |  | 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 |  |  |  |  | $ 5,409,000 | [1] | 19,313,000 | [1] |  | $ 79,353,000 | [1] | 28,267,000 | [1] |  |  |  |  |  |
Accounting Standards Update 2017-05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,806,947,000 |  |
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2017-05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in retained earnings upon adoption of accounting standard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 55,500,000 |  |
2017 Secured Financing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 227,000,000.0 |
Ground Net Lease Business | New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2017-05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale of business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 178,900,000 |  |  |  |
Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  |  |  |  |  |  |  | 40.50% |  |  |  |  | 40.50% |  |  |  |  |  |  |  |  |
Number of institutional investors | investor |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contributions by majority shareholders |  | $ 57,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of assets |  | 161,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain from discontinued operations |  |  |  |  |  |  |  | $ 123,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees waived |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,800,000 |  |  |  |  |  |  |  |  |  |
Reimbursement revenue waived |  |  |  |  |  |  |  |  |  |  |  |  | $ 800,000 |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Management Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from related party |  |  |  |  |  |  |  |  | $ 900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Expense Reimbursement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from related party |  |  |  |  |  |  |  |  | 400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Wholly-owned Subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |
Management fee, percent of equity above threshold |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.75% |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | 10b5-1 Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares purchased (in shares) | shares |  |  |  |  |  |  |  |  | 133,524 |  |  |  |  |  |  |  |  | 2,200,000 |  |  |  |  |
Aggregate value of common stock purchased |  |  |  |  |  |  |  |  | $ 2,200,000 |  |  |  |  |  |  |  |  | $ 41,700,000 |  |  |  |  |
Average share price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  | $ 16.39 |  |  |  |  |  |  |  |  | $ 18.67 |  |  |  |  |
Common stock purchase plan, number of plans | plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Ground Net Lease Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  | $ 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties | property |  |  |  |  |  |  |  | 12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar Net Lease II LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  |  |  |  |  |  |  | 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% |  |  |  |  |  |  |  |  |
Equity method investment, related party promote fee percentage |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Equity method investment, partner ownership percentage |  |  |  |  |  |  |  |  | 47.50% |  |  |  |  | 47.50% |  |  |  |  |  |  |  |  |
Net Lease Venture | Other Income | Management Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | $ 300,000 |  | 600,000 |  |  | $ 1,300,000 |  | 1,500,000 |  |  |  |  |  |  |
Ground Lease, Atlanta medical office | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  |  |  | $ 24,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan term |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loan funded |  |  |  |  |  |  |  |  | 15,500,000 |  |  |  |  | 15,500,000 |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  | 400,000 |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |  |
Ground Lease, 100 and 200 Glenridge Point | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  | $ 19,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan term |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of office buildings | office_building |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  | 600,000 |  |  |  |  | 800,000 |  |  |  |  |  |  |  |  |
Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lessor, operating lease, term of contract |  |  |  |  |  | 99 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units in real estate property to be built | unit |  |  |  |  |  | 301 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real estate, leasehold improvement allowance |  |  |  |  |  | $ 7,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward purchase contract, purchase agreement, amount |  |  |  |  |  | 34,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Great Oaks | Leasehold First Mortgage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  |  | 80,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of loans |  |  |  |  |  | $ 80,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Real Estate Equity Investments | Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments, carrying value |  |  |  |  |  |  |  |  | 74,100,000 |  |  |  |  | 74,100,000 |  |  |  |  | 38,800,000 |  |  |  |
Other Real Estate Equity Investments | Land & Development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments, carrying value |  |  |  |  |  |  |  |  | $ 70,400,000 |  |  |  |  | $ 70,400,000 |  |  |  |  | 63,800,000 |  |  |  |
Other Real Estate Equity Investments | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  |  |  |  |  |  |  | 15.50% |  |  |  |  | 15.50% |  |  |  |  |  |  |  |  |
Other Real Estate Equity Investments | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  |  |  |  |  |  |  | 95.00% |  |  |  |  | 95.00% |  |  |  |  |  |  |  |  |
Other Real Estate Equity Investment Aug 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  | $ 400,000 |  |  |  |  | $ 400,000 |  |  |  |  |  |  |  |  |
Loan commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 33,000,000 |  |  |
Loans receivable, carrying value |  |  |  |  |  |  |  |  | $ 28,900,000 |  |  |  |  | $ 28,900,000 |  |  |  |  |  |  |  |  |
Other Real Estate Equity Investment Dec 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  | $ 500,000 |  | $ 500,000 |  |  | $ 1,500,000 |  | $ 1,400,000 |  |  |  |  |  |  |
Loan commitments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 27,000,000
Loans receivable, carrying value |  |  |  |  |  |  |  |  | $ 27,000,000 |  |  |  |  | 27,000,000 |  |  |  |  | $ 25,400,000 |  |  |  |
Subsequent Event | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock received as payment of management fee | shares |  |  | 45,941 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in noncontrolling interest from consolidation of equity method investment |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 188,279,000 |  |  |  |  |  |  |  |  |
[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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