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Other Investments (Safehold Inc). (Details) $ / shares in Units, ft² in Millions |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Jan. 02, 2019 director | Sep. 30, 2021 USD ($) $ / shares shares | Jul. 31, 2021 USD ($) | Jun. 30, 2021 USD ($) ft² agreement | Feb. 28, 2021 USD ($) | Jan. 31, 2021 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2020 | Mar. 31, 2020 USD ($) shares | May 31, 2019 | Jan. 31, 2019 USD ($) $ / shares shares | Oct. 31, 2017 USD ($) | Sep. 30, 2021 USD ($) $ / shares | Sep. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) | Sep. 30, 2021 USD ($) $ / shares shares | Sep. 30, 2020 USD ($) | Mar. 31, 2021 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of limited partnership units purchased (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 12,500,000 |  |  |  |  |  |  |
Investor units, purchase price per unit (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  | $ 20.00 |  |  |  |  |  |  |
Cash investment in equity method investments |  |  |  |  |  |  |  |  |  |  | $ 250,000,000.0 |  |  |  |  | $ 171,005,000 | $ 194,775,000 |
Conversion of stock |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |
Restriction period on common stock received as payment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years |  |
Management agreement, termination fee as multiple of prior year management fee |  | 3 |  |  |  |  |  |  |  |  |  |  | 3 |  |  | 3 |  |
Income from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  | $ 25,611,000 | $ 6,055,000 |  | $ 28,433,000 | 6,118,000 |
Payment to acquire purchase option agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000.0 |  |  |
Expenses incurred in acquisition of purchase option agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200,000 |  |  |
SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense reimbursements from related party |  |  |  |  |  |  |  |  |  |  |  |  | 1,900,000 | 1,300,000 |  | 5,600,000 | 3,800,000 |
Proceeds from sale of purchase option agreement |  |  |  | $ 1,200,000 |  |  |  |  |  |  |  |  |  |  | 1,200,000 |  |  |
Gain recognized on sale of purchase option agreement |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of agreements | agreement |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale price per agreement for sale of land and related Ground Lease, subject to certain conditions being met |  |  |  | $ 42,000,000.0 |  |  |  |  |  |  |  |  |  |  | $ 42,000,000.0 |  |  |
Multiple required that may trigger receipt of additional consideration |  |  |  | 1.25 |  |  |  |  |  |  |  |  |  |  | 1.25 |  |  |
Percentage of required return on investment that may trigger receipt of additional consideration |  |  |  | 9.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management Fees | SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee revenue from related party |  |  |  |  |  |  |  |  |  |  |  |  | 3,600,000 | 3,200,000 |  | $ 10,600,000 | 9,300,000 |
Maximum | SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leasehold improvement allowance, Ground Leases |  |  |  | $ 83,000,000.0 |  |  |  |  |  |  |  |  |  |  | $ 83,000,000.0 |  |  |
Fee percent of SAFE Equity up to $1.5 billion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee as percent of equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |
Fee percent of SAFE Equity up to $1.5 billion | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee equity threshold amount |  | $ 1,500,000,000 |  |  |  |  |  |  |  |  |  |  | 1,500,000,000 |  |  | $ 1,500,000,000 |  |
Fee percent of SAFE Equity between $1.5 billion and $3.0 billion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee as percent of equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.25% |  |
Fee percent of SAFE Equity between $1.5 billion and $3.0 billion | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee equity threshold amount |  | 3,000,000,000.0 |  |  |  |  |  |  |  |  |  |  | 3,000,000,000.0 |  |  | $ 3,000,000,000.0 |  |
Fee percent of SAFE Equity between $1.5 billion and $3.0 billion | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee equity threshold amount |  | 1,500,000,000 |  |  |  |  |  |  |  |  |  |  | 1,500,000,000 |  |  | $ 1,500,000,000 |  |
Fee percent of SAFE Equity between $3.0 billion and $5.0 billion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee as percent of equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.375% |  |
Fee percent of SAFE Equity between $3.0 billion and $5.0 billion | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee equity threshold amount |  | 5,000,000,000.0 |  |  |  |  |  |  |  |  |  |  | 5,000,000,000.0 |  |  | $ 5,000,000,000.0 |  |
Fee percent of SAFE Equity between $3.0 billion and $5.0 billion | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee equity threshold amount |  | 3,000,000,000.0 |  |  |  |  |  |  |  |  |  |  | 3,000,000,000.0 |  |  | $ 3,000,000,000.0 |  |
Fee percent of SAFE Equity over $5.0 billion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee as percent of equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.50% |  |
Fee percent of SAFE Equity over $5.0 billion | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee equity threshold amount |  | $ 5,000,000,000.0 |  |  |  |  |  |  |  |  |  |  | $ 5,000,000,000.0 |  |  | $ 5,000,000,000.0 |  |
Safehold Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase option, purchase price to acquire property |  |  |  | $ 215,000,000.0 |  |  |  |  |  |  |  |  |  |  | 215,000,000.0 |  |  |
Purchase option, square feet of future office space development | ft² |  |  |  | 1.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safehold Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest |  | 63.60% |  |  |  |  |  |  |  |  |  |  | 63.60% |  |  | 63.60% |  |
Cash investment in equity method investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 27,900,000 |  |
Shares acquired (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 |  |
Average cost per share | $ / shares |  | $ 71.68 |  |  |  |  |  |  |  |  |  |  | $ 71.68 |  |  | $ 71.68 |  |
Safehold Inc. | Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash investment in equity method investments |  | $ 50,000,000.0 |  |  |  |  |  |  | $ 80,000,000.0 |  |  |  |  |  |  |  |  |
Shares acquired (in shares) | shares |  | 657,894 |  |  |  |  |  |  | 1,700,000 |  |  |  |  |  |  |  |  |
Safehold Inc. | Real estate equity investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Voting power cap threshold | 41.90% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of director nominees | director | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Multifamily Development San Jose |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lessor, operating lease, term of contract (in years) |  |  |  |  |  |  |  |  |  |  |  | 99 years |  |  |  |  |  |
Leasehold improvement allowance, Ground Leases |  |  |  |  |  |  |  |  |  |  |  | $ 7,200,000 |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  | 900,000 |  |  | 2,500,000 |
Proceeds from sale of ground lease |  |  |  |  |  |  | $ 34,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  |  | $ 6,100,000 |  |  |  |  |  |  |  |  |  |  |
Multifamily Development San Jose | 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 |  |  |  |  |  |
Ground Lease, Washington DC building conversion |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  |  |  |  |  |  |  | $ 13,300,000 |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  | $ 800,000 |
Other Real Estate Equity Investment January 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest |  |  |  |  |  | 53.00% |  |  |  |  |  |  |  |  |  |  |  |
Equity method investments |  |  |  |  |  | $ 63,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Recognized gain or loss |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |
Other Real Estate Equity Investment September 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest |  | 53.00% |  |  |  |  |  |  |  |  |  |  | 53.00% |  |  | 53.00% |  |
Cash investment in equity method investments |  | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  | 75,000,000.0 |  |  |  |  |  |  |  |  |  |  | $ 75,000,000.0 |  |  | $ 75,000,000.0 |  |
Multi-Family Project |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 75,000,000.0
Income from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |
Consideration received |  | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Hotel Property |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  | $ 50,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  | $ 400,000 |  |  | $ 2,900,000 |  |
Consideration received |  |  |  |  | $ 1,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Recognized gain or loss |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ground Lease, Honolulu, Hawaii [Member] | iStar Net Lease II LLC (Net Lease Venture II) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lessor, operating lease, term of contract (in years) |  |  |  |  |  |  |  | 99 years |  |  |  |  |  |  |  |  |  |
Land Parcel One | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leasehold improvement allowance, Ground Leases |  |  |  | $ 83,000,000.0 |  |  |  |  |  |  |  |  |  |  | 83,000,000.0 |  |  |
Land Parcel Two | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leasehold improvement allowance, Ground Leases |  |  |  | $ 83,000,000 |  |  |  |  |  |  |  |  |  |  | $ 83,000,000 |  |  |

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