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