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Other Investments - Safehold Inc. (Details) $ / shares in Units, shares in Millions |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |
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Jan. 02, 2019 director | Jun. 30, 2020 | Mar. 31, 2020 USD ($) shares | Aug. 31, 2019 | Feb. 28, 2019 | Jan. 31, 2019 USD ($) $ / shares shares | May 31, 2018 USD ($) office_building | Oct. 31, 2017 USD ($) | Sep. 30, 2020 USD ($) multiple | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) multiple | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) | Aug. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (as a percent) |  |  |  |  |  |  |  |  |  | 50.00% |  | 50.00% |  |
Number of limited partnership units purchased (in shares) | shares |  |  |  |  |  | 12.5 |  |  |  |  |  |  |  |
Investor units, purchase price per unit (in dollars per share) | $ / shares |  |  |  |  |  | $ 20.00 |  |  |  |  |  |  |  |
Additional cash investment in equity method investment |  |  |  |  |  | $ 250,000,000.0 |  |  |  |  | $ 194,775,000 | $ 494,339,000 |  |
Restriction period on common stock received as payment |  |  |  |  |  |  |  |  |  |  | 2 years |  |  |
Management agreement, termination fee as multiple of prior year management fee | multiple |  |  |  |  |  |  |  |  | 3 |  | 3 |  |  |
Commitment to provide loan to lessee |  |  |  |  |  |  |  |  |  |  |  |  | $ 150,000,000.0 |
SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ground lease term (in years) |  | 99 years |  | 99 years | 98 years |  |  |  |  |  |  | 99 years |  |
SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (as a percent) |  |  |  |  |  |  |  |  | 65.80% |  | 65.80% |  |  |
Ground Lease, Atlanta medical office |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 24,000,000.0
Amount of loan funded |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 18,400,000
Interest income |  |  |  |  |  |  |  |  |  | $ 300,000 |  | $ 1,200,000 |  |
Multifamily Development San Jose |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of loan funded |  |  |  |  |  |  |  |  | $ 59,100,000 |  | $ 59,100,000 |  |  |
Interest income |  |  |  |  |  |  |  |  | 900,000 | 400,000 | 2,500,000 | 700,000 |  |
Lessor, operating lease, term of contract (in years) |  |  |  |  |  |  |  | 99 years |  |  |  |  |  |
Real estate, leasehold improvement allowance |  |  |  |  |  |  |  | $ 7,200,000 |  |  |  |  |  |
Proceeds from sale of ground lease |  |  |  |  |  |  |  |  | 34,000,000.0 |  | 34,000,000.0 |  |  |
Gain on sale of ground lease |  |  |  |  |  |  |  |  |  |  | 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, Multi-tenant Office Atlanta |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to provide loan to lessee |  |  |  |  |  |  | $ 19,900,000 |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  | 500,000 |  | 1,600,000 |  |
Number of office buildings | office_building |  |  |  |  |  |  | 2 |  |  |  |  |  |  |
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 | 300,000 | 800,000 | 700,000 |  |
SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense reimbursements from related party |  |  |  |  |  |  |  |  | 1,300,000 | 500,000 | 3,800,000 | 1,600,000 |  |
Management Fees | SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee revenue from related party |  |  |  |  |  |  |  |  | 3,200,000 | $ 1,900,000 | $ 9,300,000 | $ 5,000,000.0 |  |
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 |  |  |
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 |  |  |
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 |  |  |
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 |  |  |
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 |  |  |
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 | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fee equity threshold amount |  |  |  |  |  |  |  |  | $ 5,000,000,000.0 |  | $ 5,000,000,000.0 |  |  |
SAFE | Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares acquired (in shares) | shares |  |  | 1.7 |  |  |  |  |  |  |  |  |  |  |
Value of common stock issued in private placement |  |  | $ 80,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Real estate equity investments | SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Voting power cap threshold (as a percent) | 41.90% |  |  |  |  |  |  |  |  |  |  |  |  |
Number of director nominees | director | 3 |  |  |  |  |  |  |  |  |  |  |  |  |
Independent period | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |
Standstill provisions (in years) | 2 years |  |  |  |  |  |  |  |  |  |  |  |  |
Exchange shares restriction period | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |
Stock transfer limitation maximum per agreement, other than widely distributed public offering (as a percent) | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |

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