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Other Investments (Narrative) (Details) $ / shares in Units, shares in Thousands |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Jun. 27, 2017 USD ($) | Apr. 14, 2017 USD ($) | Aug. 31, 2017 USD ($) | Apr. 30, 2017 USD ($) property | Dec. 31, 2016 USD ($) | Jun. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) unit $ / shares shares | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) unit | Sep. 30, 2016 USD ($) office | Mar. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net proceeds from sales of other investments |  |  |  |  |  |  |  |  | $ 0 | $ 39,810,000 |
Distributions from operations of other investments |  |  |  |  |  |  |  |  | 39,076,000 | 44,893,000 |
Earnings from equity method investments |  |  |  |  |  |  | $ 2,461,000 | $ 26,540,000 | 13,677,000 | 74,254,000 |
Acquisitions of real estate assets |  |  |  |  |  |  |  |  | 0 | 4,740,000 |
Net proceeds from sales of real estate |  |  |  |  |  |  |  |  | 201,939,000 | 412,335,000 |
Payments to acquire equity method investments |  |  |  |  |  |  |  |  | 181,279,000 | 45,635,000 |
Noncontrolling interest, ownership percentage by parent |  |  |  |  | 95.70% |  |  |  |  |  |
Carrying value of assets |  |  |  |  | $ 4,825,514,000 |  | 5,795,219,000 |  | 5,795,219,000 |  |
Gain from discontinued operations |  |  |  |  |  |  | 0 | 0 | 123,418,000 | 0 |
Equity method investments, carrying value |  |  |  |  | 214,406,000 |  | 289,037,000 |  | 289,037,000 |  |
Cost method investments |  |  |  |  | 1,400,000 |  | 800,000 |  | 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 |  |  |  |  | 3,600,000 |  | 21,800,000 |  | 21,800,000 |  |
Loan commitments |  |  |  |  | 27,000,000 |  |  |  |  |  |
Loans receivable, carrying value |  |  |  |  | 22,700,000 |  | 24,300,000 |  | 24,300,000 |  |
Interest income |  |  |  |  |  |  | 500,000 |  | 1,400,000 |  |
Other real estate equity investments | Land & Development |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Equity method investments, carrying value |  |  |  |  | 49,600,000 |  | $ 57,900,000 |  | $ 57,900,000 |  |
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% |  |
Oak Hill Funds |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Earnings from equity method investments |  |  |  |  |  |  |  | 600,000 |  | $ 4,300,000 |
Net lease venture |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  | 51.90% |  | 51.90% |  |
Partners' capital account, contributions (up to) |  |  |  |  |  |  | $ 500,000,000 |  | $ 500,000,000 |  |
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% |  |
Acquisitions of real estate assets |  |  |  |  |  |  |  |  | $ 59,000,000 |  |
Net proceeds from sales of real estate |  |  |  |  |  |  |  |  | 6,200,000 |  |
Financing derecognized after sale |  |  |  |  |  |  |  |  | 18,900,000 |  |
Number of properties | office |  |  |  |  |  |  |  |  |  | 2 |
Distributions received from Net Lease Venture |  |  |  |  |  |  |  |  | 23,700,000 | $ 3,900,000 |
Total assets carrying value |  |  |  |  | 511,300,000 |  | $ 635,100,000 |  | 635,100,000 |  |
Payments to acquire equity method investments |  |  |  |  |  |  |  |  | 37,700,000 | 35,600,000 |
Net lease venture | Other income |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Management fees revenue |  |  |  |  |  |  | $ 600,000 | 400,000 | $ 1,500,000 | 1,200,000 |
Marina Palms |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  | 47.50% |  | 47.50% |  |
Total assets carrying value |  |  |  |  | $ 201,800,000 |  | $ 43,500,000 |  | $ 43,500,000 |  |
Number of units | unit |  |  |  |  |  |  | 468 |  | 468 |  |
Marine Palms North Tower |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of units | unit |  |  |  |  |  |  | 234 |  | 234 |  |
Number of units remaining for sale | unit |  |  |  |  |  |  | 1 |  | 1 |  |
Marine Palms South Tower |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of units | unit |  |  |  |  |  |  | 234 |  | 234 |  |
Percentage of units pre-sold |  |  |  |  |  |  |  |  | 85.00% |  |
Newly formed unconsolidated entity |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  | 50.00% |  |  |  |  |  |
Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  | 35.00% |  | 35.00% |  |
Contributions by majority shareholders |  | $ 57,500,000 |  |  |  |  |  |  |  |  |
Carrying value of assets |  | 161,100,000 |  |  |  |  |  |  |  |  |
Gain from discontinued operations |  |  |  | $ 123,400,000 |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Ground Lease [Member] |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  | $ 24,000,000 |  |  |  |  |  |  |  |
Loan term |  |  | 1 year |  |  |  |  |  |  |  |
Amount of loan funded |  |  |  |  |  |  | $ 5,100,000 |  | $ 5,100,000 |  |
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. | 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 |  |  |  |  |  |  |  |  |  |
Wholly-owned Subsidiary | Other real estate equity investments |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net proceeds from sales of other investments |  |  |  |  |  | $ 39,800,000 |  |  |  |  |
Gain on sale of equity method investment |  |  |  |  |  | 31,500,000 |  |  |  |  |
Gain on sale of equity method investment attributable to noncontrolling interest |  |  |  |  |  | $ 10,100,000 |  |  |  |  |
Distributions from operations of other investments |  |  |  |  |  |  |  | 15,800,000 |  | 11,600,000 |
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, percent of equity above threshold |  |  |  |  |  |  |  |  | 0.75% |  |
Management fee, shareholders' equity threshold amount |  |  |  |  |  |  | 2,500,000,000.0 |  | $ 2,500,000,000.0 |  |
Ground Net Lease Business | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net proceeds from sales of real estate |  | 113,000,000 |  | $ 113,000,000 |  |  |  |  |  |  |
Number of properties | property |  |  |  | 12 |  |  |  |  |  |  |
Payments to acquire equity method investments |  | $ 55,500,000 |  | $ 55,500,000 |  |  |  |  |  |  |
Noncontrolling interest (as a percent) |  | 49.00% |  |  |  |  |  |  |  |  |
Noncontrolling interest, ownership percentage by parent |  | 51.00% |  |  |  |  |  |  |  |  |
2017 Secured Financing |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  |  |  |  |  | $ 227,000,000.0
Real estate equity investments |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Earnings from equity method investments |  |  |  |  |  |  | 1,851,000 | 23,194,000 | 12,461,000 | 65,383,000 |
Equity method investments, carrying value |  |  |  |  | $ 181,056,000 |  | 270,313,000 |  | 270,313,000 |  |
Real estate equity investments | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Earnings from equity method investments |  |  |  |  |  |  |  | 0 | 388,000 | 0 |
Equity method investments, carrying value |  |  |  |  | 0 |  | 75,023,000 |  | 75,023,000 |  |
Real estate equity investments | Other real estate equity investments |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Earnings from equity method investments |  |  |  |  |  |  | 55,000 | 16,289,000 | 4,304,000 | 43,187,000 |
Equity method investments, carrying value |  |  |  |  | 53,202,000 |  | 79,768,000 |  | 79,768,000 |  |
Real estate equity investments | Net lease venture |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Earnings from equity method investments |  |  |  |  |  |  | 962,000 | 723,000 | 2,975,000 | 2,613,000 |
Equity method investments, carrying value |  |  |  |  | 92,669,000 |  | 110,153,000 |  | 110,153,000 |  |
Real estate equity investments | Marina Palms |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Earnings from equity method investments |  |  |  |  |  |  | 494,000 | $ 6,182,000 | 4,794,000 | $ 19,583,000 |
Equity method investments, carrying value |  |  |  |  | $ 35,185,000 |  | 5,369,000 |  | $ 5,369,000 |  |
Real estate equity investments | Safety, Income and Growth, Inc. | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Earnings from equity method investments |  |  |  |  |  |  | $ 340,000 |  |  |  |
10b5-1 Plan | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of shares purchased | shares |  |  |  |  |  |  | 1,300 |  |  |  |
Aggregate value of common stock purchased |  |  |  |  |  |  | $ 24,500,000 |  |  |  |
Average share price (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 19.20 |  |  |  |
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 (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 19.20 |  |  |  |

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