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Real Estate (Dispositions) (Details) |  | 1 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jun. 27, 2017 USD ($) | Apr. 30, 2017 USD ($) lease property | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Jun. 30, 2017 USD ($) | Dec. 31, 2016 USD ($) lease_asset property | Dec. 31, 2015 USD ($) property | Dec. 31, 2014 USD ($) lease_asset | Feb. 28, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Net proceeds from sales of real estate |  |  |  |  |  | $ 435,560,000 | $ 362,530,000 | $ 404,336,000 |
Number of Net Lease Assets | lease_asset |  |  |  |  |  | 1 |  |  |
Real estate, net |  |  | $ 1,387,274,000 | $ 1,490,775,000 |  | $ 1,387,274,000 | 1,490,775,000 |  |
Cash Provided by (Used in) Operating Activities, Discontinued Operations |  |  |  |  |  | 16,662,000 | 14,446,000 | 15,459,000 |
Income from sales of real estate |  |  |  |  |  | $ 105,296,000 | 93,816,000 | 89,943,000 |
Noncontrolling interest, ownership percentage by parent |  |  | 95.70% |  |  | 95.70% |  |  |
Cash Provided by (Used in) Investing Activities, Discontinued Operations |  |  |  |  |  | $ 7,972,000 | 0 | 0 |
Deferred Expenses and Other Assets Net |  |  | $ 162,112,000 | 142,484,000 |  | 162,112,000 | 142,484,000 |  |
Deferred operating lease income receivable, net |  |  | 88,189,000 | 93,424,000 |  | $ 88,189,000 | 93,424,000 |  |
Number of properties acquired | property |  |  |  |  |  | 2 |  |  |
Ground Net Lease Business |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Real estate, net |  |  | 104,500,000 | 103,200,000 |  | $ 104,500,000 | 103,200,000 |  |
Deferred Expenses and Other Assets Net |  |  | 37,500,000 | 39,000,000 |  | 37,500,000 | 39,000,000 |  |
Deferred operating lease income receivable, net |  |  | 8,200,000 | 4,000,000 |  | 8,200,000 | 4,000,000 |  |
Condominium Units |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from sale of other real estate |  |  |  |  |  | 97,800,000 | 127,900,000 | 236,200,000 |
Income from sales of real estate |  |  |  |  |  | 26,100,000 | 40,100,000 | 79,100,000 |
Net Lease Asset Two |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  | 21,100,000 | 40,100,000 | 6,200,000 |
Net proceeds from sales of real estate |  |  |  |  |  | 117,200,000 | 100,800,000 | 127,200,000 |
Operating Properties |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Real estate, net |  |  | 476,162,000 | 481,504,000 |  | 476,162,000 | 481,504,000 |  |
Property sold, aggregate, carrying value |  |  |  |  |  |  | 126,300,000 |  |
Income from sales of real estate |  |  |  |  |  |  |  | 4,600,000 |
Net proceeds from sales of real estate |  |  |  |  |  |  | $ 93,500,000 | 34,200,000 |
Operating leases, term |  |  |  |  |  |  | 99 years |  |
Other finite-lived intangible assets |  |  |  | 32,800,000 |  |  | $ 32,800,000 |  |
Payments for repurchase of redeemable noncontrolling interest |  |  |  | $ 6,400,000 |  |  |  |  |
Number of real estate properties sold | property |  |  |  |  |  |  | 3 |  |
Operating Properties | RCC |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  | 49,300,000 | $ 13,600,000 |  |
Sales price of real estate held for investment |  |  |  |  |  | 229,100,000 | $ 68,500,000 |  |
iStar's ownership percentage |  |  |  | 50.00% |  |  | 50.00% |  |
Net Lease |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Real estate, net |  |  | 911,112,000 | $ 1,009,271,000 |  | $ 911,112,000 | $ 1,009,271,000 |  |
Net Lease | iStar Net Lease I LLC |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
iStar's ownership percentage |  |  |  |  |  |  |  |  | 51.90%
Net proceeds from sales of real estate |  |  |  |  |  |  |  | $ 10,100,000 |
Noncontrolling interest, ownership percentage by parent |  |  |  |  |  |  |  | 72.00% |
Transfer mortgage payable |  |  |  |  |  |  |  | $ 26,000,000 |
Number of properties acquired | lease_asset |  |  |  |  |  |  |  | 58 |
Operating Properties |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Property sold, aggregate, carrying value |  |  |  |  |  |  | 5,000,000 |  |
Operating Properties | RCC |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  | 8,800,000 |  |  |  | 13,600,000 |  |
Sales price of real estate held for investment |  |  | $ 36,000,000 |  |  |  | $ 68,500,000 |  |
iStar's ownership percentage |  |  | 50.00% | 50.00% |  | 50.00% | 50.00% |  |
Net Lease Asset One | iStar Net Lease I LLC |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Net proceeds from sales of real estate |  |  |  |  |  |  |  | $ 93,700,000 |
Ground Net Lease Business |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Revenue |  |  |  |  |  | $ 21,839,000 | $ 18,520,000 | 16,878,000 |
Disposal Group, Including Discontinued Operation, Operating Expense |  |  |  |  |  | 3,569,000 | 3,443,000 | 3,756,000 |
Disposal Group, Including Discontinued Operation, Operating Income (Loss) |  |  |  |  |  | $ 18,270,000 | $ 15,077,000 | $ 13,122,000 |
Subsequent Event | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Property, Plant and Equipment, Net |  | $ 161,100,000 |  |  |  |  |  |  |
Subsequent Event | Ground Net Lease Business | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Net proceeds from sales of real estate |  | $ 113,000,000 |  |  |  |  |  |  |
Number of Net Lease Assets | lease |  | 7 |  |  |  |  |  |  |
Payments to Acquire Equity Method Investments |  | $ 55,500,000 |  |  |  |  |  |  |
Number of properties acquired | property |  | 12 |  |  |  |  |  |  |
Number of Master Lease Assets | lease |  | 1 |  |  |  |  |  |  |
Debt Instrument, Number of Properties Covered Under Master Lease Agreement | property |  | 5 |  |  |  |  |  |  |
Subsequent Event | Private Placement [Member] | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Proceeds from Issuance Initial Public Offering | $ 205,000,000 |  |  |  |  |  |  |  |
Proceeds from Issuance of Private Placement | 45,000,000 |  |  |  |  |  |  |  |
Subsequent Event | IPO [Member] | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Stock Issuance Costs | $ 25,000,000 |  |  |  |  |  |  |  |
Subsequent Event | 2017 Secured Financing |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 227,000,000.0 |  |  |  |
Subsequent Event | Subsidiaries | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Related Party Transaction, Management Fee, Percent of Equity Below Threshold |  |  |  |  | 1.00% |  |  |  |

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