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Other Investments (Narrative) (Details) $ / shares in Units, shares in Thousands |  |  | 1 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 27, 2017 USD ($) | Apr. 14, 2017 USD ($) | Oct. 31, 2017 USD ($) unit | Aug. 31, 2017 USD ($) | Apr. 30, 2017 USD ($) property | Dec. 31, 2016 USD ($) | Nov. 30, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2015 USD ($) | Dec. 31, 2017 USD ($) unit plan $ / shares shares | Dec. 31, 2017 USD ($) unit | Dec. 31, 2016 USD ($) property | Dec. 31, 2015 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2014 USD ($) | Feb. 28, 2014 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions of real estate assets |  |  |  |  |  |  |  |  |  |  | $ 6,600,000 | $ 38,433,000 | $ 0 |  |  |
Net proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  | 314,013,000 | $ 435,560,000 | 362,530,000 |  |  |
Number of properties acquired | property |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |
Contributions to other investments |  |  |  |  |  |  |  |  |  |  | 224,219,000 | $ 58,197,000 | 11,531,000 |  |  |
Total assets |  |  |  |  |  | $ 2,803,411,000 |  |  |  | $ 2,493,798,000 | 2,493,798,000 | 2,803,411,000 |  |  |  |
Assets |  |  |  |  |  | 4,825,514,000 |  |  |  | 4,731,078,000 | 4,731,078,000 | 4,825,514,000 |  |  |  |
Gain from discontinued operations |  |  |  |  |  |  |  |  |  |  | 123,418,000 | 0 | 0 |  |  |
Debt obligation |  |  |  |  |  | 3,418,357,000 |  |  |  | 3,539,991,000 | 3,539,991,000 | 3,418,357,000 |  |  |  |
Total gross carrying value of loans |  |  |  |  |  | 1,456,068,000 |  |  |  | 1,289,568,000 | 1,289,568,000 | 1,456,068,000 |  |  |  |
Carrying value |  |  |  |  |  | 214,406,000 |  |  |  | 321,241,000 | 321,241,000 | 214,406,000 |  |  |  |
Income from sales of real estate |  |  |  |  |  |  |  |  |  |  | 92,049,000 | 105,296,000 | 93,816,000 |  |  |
Distributions from other investments |  |  |  |  |  |  |  |  |  |  | 49,672,000 | 92,482,000 | 119,854,000 |  |  |
Earnings (loss) from equity method investments |  |  |  |  |  |  |  |  |  |  | 13,015,000 | 77,349,000 | 32,153,000 |  |  |
Cost investments |  |  |  |  |  | 1,400,000 |  |  |  | 800,000 | 800,000 | 1,400,000 |  |  |  |
Proceeds from sale of available-for-sale securities |  |  |  |  |  |  |  |  |  |  |  |  | 7,400,000 |  |  |
Available-for-sale securities, realized gain |  |  |  |  |  |  |  |  |  |  |  |  | 2,600,000 |  |  |
2017 Secured Financing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 227,000,000.0 |  |
Net Lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  |  |  |  |  |  | 87,500,000 | 21,100,000 | 40,100,000 |  |  |
Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  |  |  |  |  |  | 4,500,000 | 75,400,000 | 53,700,000 |  |  |
iStar Net Lease I LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions of real estate assets |  |  |  |  |  |  |  |  |  |  | 59,000,000 |  |  |  |  |
Net proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  | 6,200,000 |  |  |  |  |
Financing derecognized during sale |  |  |  |  |  |  |  |  |  |  | 18,900,000 |  |  |  |  |
Payments to acquire equity method investments |  |  |  |  |  |  |  |  |  |  | 49,200,000 |  |  |  |  |
Contributions to other investments |  |  |  |  |  |  |  |  |  |  |  | 37,700,000 |  |  |  |
Distributions from other investments |  |  |  |  |  |  | $ 13,200,000 |  | $ 61,200,000 |  | 26,000,000 |  |  |  |  |
iStar Net Lease I LLC | Net Lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  |  |  |  |  | 5 years |  | 10 years |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  | $ 29,000,000.0 |  | $ 120,000,000.0 |  |  |  |  |  |  |
iStar Net Lease I LLC | Net Lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 51.90%
Partners' capital account, contributions (up to) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000
Equity method investment, related party ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.60%
Equity method investment, related party promote fee percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00%
Equity method investment, partner ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 47.50%
Total assets |  |  |  |  |  | 511,300,000 |  |  |  | $ 658,300,000 | 658,300,000 | 511,300,000 |  |  |  |
iStar Net Lease I LLC | Other Income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees revenue |  |  |  |  |  |  |  |  |  |  | $ 2,100,000 | 1,600,000 | $ 1,500,000 |  |  |
Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating leases, term |  |  | 99 years |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units in real estate property to be built | unit |  |  | 301 |  |  |  |  |  |  |  |  |  |  |  |  |
Leasehold improvement allowance |  |  | $ 7,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Marina Palms |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  |  | 47.50% | 47.50% |  |  |  |  |
Number of units | unit |  |  |  |  |  |  |  |  |  | 468 | 468 |  |  |  |  |
Marina Palms | Land |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total assets |  |  |  |  |  | 201,800,000 |  |  |  | $ 32,400,000 | $ 32,400,000 | 201,800,000 |  |  |  |
Marine Palms, LLC, North Tower |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units for sale | unit |  |  |  |  |  |  |  |  |  | 1 | 1 |  |  |  |  |
Number of units | unit |  |  |  |  |  |  |  |  |  | 234 | 234 |  |  |  |  |
Marine Palms, LLC, South Tower |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units | unit |  |  |  |  |  |  |  |  |  | 234 | 234 |  |  |  |  |
Percentage of units pre-sold |  |  |  |  |  |  |  |  |  |  | 85.00% |  |  |  |  |
Other real estate equity investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings (loss) from equity method investments |  |  |  |  |  |  |  |  |  |  |  | 11,600,000 |  |  |  |
Other real estate equity investments | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  |  | 20.00% | 20.00% |  |  |  |  |
Other real estate equity investments | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  |  | 95.00% | 95.00% |  |  |  |  |
Other real estate equity investments | Land |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value |  |  |  |  |  | 49,600,000 |  |  |  | $ 61,300,000 | $ 61,300,000 | 49,600,000 |  |  |  |
Other real estate equity investments | Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value |  |  |  |  |  | 3,600,000 |  |  |  | $ 38,800,000 | $ 38,800,000 | 3,600,000 |  |  |  |
RCC | Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  |  | 50.00% | 50.00% |  | 50.00% |  |  |
Contributions to other investments |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,600,000 |  |  |
Sales price of real estate held for investment |  |  |  |  |  | 36,000,000 |  |  |  |  |  |  | 68,500,000 |  |  |
Income from sales of real estate |  |  |  |  |  | 8,800,000 |  |  |  |  |  |  | 13,600,000 |  |  |
Proceeds from sale of other real estate, net of equity contribution |  |  |  |  |  | 7,000,000 |  |  |  |  |  |  | 55,400,000 |  |  |
Loans commitment |  |  |  |  |  |  |  |  |  | $ 27,000,000 | $ 27,000,000 |  |  |  |  |
Payments for funding under loan commitments |  |  |  |  |  |  |  |  |  | $ 25,400,000 | 25,400,000 |  |  |  |  |
Distributions from other investments |  |  |  |  |  |  |  |  |  |  | $ 17,600,000 |  | 21,000,000 |  |  |
Equity Method Investee Two | Land |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  |  | 50.00% | 50.00% |  |  |  | 85.70% |
Carrying value |  |  |  |  |  | $ 0 |  |  |  |  |  | 0 |  |  |  |
Loans commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 45,700,000 |
Earnings (loss) from equity method investments |  |  |  |  |  |  |  | $ 11,600,000 |  |  |  |  |  |  |  |
Interest and fee income |  |  |  |  |  |  |  |  |  |  |  | $ 3,600,000 | $ 3,900,000 |  |  |
Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  |  | 37.60% | 37.60% |  |  |  |  |
Payments to acquire interest in joint venture |  | $ 57,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets |  | 161,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain from discontinued operations |  |  |  |  | $ 123,400,000 |  |  |  |  |  |  |  |  |  |  |
Maximum allowable ownership interest (as a percent) |  |  |  |  |  |  |  |  |  | 39.90% |  |  |  |  |  |
Income from sales of real estate |  |  |  |  | 123,400,000 |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Ground Lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |
Commitment to extend credit to customer |  |  |  | $ 24,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Commitment to extend credit, amount funded |  |  |  |  |  |  |  |  |  | $ 5,200,000 | $ 5,200,000 |  |  |  |  |
Safety, Income and Growth, Inc. | Ground Net Lease Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from sales of real estate |  | 113,000,000 |  |  | 113,000,000 |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  | $ 55,500,000 |  |  | $ 55,500,000 |  |  |  |  |  |  |  |  |  |  |
Number of properties acquired | property |  |  |  |  | 12 |  |  |  |  |  |  |  |  |  |  |
Ownership interest by noncontrolling owners (as a percent) |  | 49.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 51.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Private Placement | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
IPO | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments of stock issuance costs |  |  |  |  |  |  |  |  |  | $ 18,900,000 |  |  |  |  |  |
Offering costs | $ 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
10b5-1 Plan | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares purchased | shares |  |  |  |  |  |  |  |  |  | 1,800 |  |  |  |  |  |
Aggregate value of common stock purchased |  |  |  |  |  |  |  |  |  | $ 34,100,000 |  |  |  |  |  |
Average share price of common stock purchased (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 18.85 |  |  |  |  |  |
Number of common stock purchase 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 |  |  |  |  |  |
Aggregate value of common stock purchased |  |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |  |  |
Average share price of common stock purchased (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 19.20 |  |  |  |  |  |
Subsidiaries | 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 |  |  |  |  |
Management fee, percent of equity above threshold |  |  |  |  |  |  |  |  |  | 0.75% |  |  |  |  |  |
Leasehold First Mortgage | Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt obligation |  |  | 80,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Total gross carrying value of loans |  |  | $ 80,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward purchase contract, purchase agreement, amount |  |  |  |  |  |  |  |  |  | $ 34,000,000 | $ 34,000,000 |  |  |  |  |

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