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Other Investments (Narrative) (Details) |  |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended | 13 Months Ended | 18 Months Ended |  |  |  |
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Jun. 27, 2017 USD ($) | Apr. 14, 2017 USD ($) | Dec. 31, 2018 USD ($) ft² lease building | Jun. 30, 2018 USD ($) lease facility | May 31, 2018 USD ($) office_building | Aug. 31, 2017 USD ($) | Apr. 30, 2017 USD ($) property | Dec. 31, 2016 USD ($) | Nov. 30, 2016 USD ($) | Feb. 25, 2019 shares | Dec. 31, 2018 USD ($) ft² shares | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) ft² | Jun. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | Jan. 02, 2018 USD ($) | Dec. 31, 2018 USD ($) ft² $ / shares shares | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) property | Dec. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) ft² $ / shares shares | Aug. 31, 2018 USD ($) | Jan. 01, 2018 USD ($) | Oct. 31, 2017 USD ($) unit | Feb. 28, 2014
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 94,578,000 | $ 224,219,000 | $ 58,197,000 |  |  |  |  |  |
Carrying value of assets |  |  | $ 5,014,277,000 |  |  |  |  |  |  |  | $ 5,014,277,000 |  |  |  | $ 4,731,078,000 |  |  |  | $ 5,014,277,000 |  | $ 4,731,078,000 |  | 5,014,277,000 | 4,731,078,000 |  | $ 4,731,078,000 | $ 5,014,277,000 |  | $ 4,806,947,000 |  |
Net proceeds from sales of real estate |  |  |  | $ 36,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 411,786,000 | 314,013,000 | $ 435,560,000 |  |  |  |  |  |
Number of properties | property |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |
Gain on sale of discontinued operation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | 123,418,000 | $ 0 |  |  |  |  |  |
Increase to retained earnings upon adoption of new accounting standard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 75,869,000 |  |
Equity interest (percent) |  |  | 51.90% |  |  |  |  |  |  |  | 51.90% |  |  |  |  |  |  |  | 51.90% |  |  |  | 51.90% |  |  |  | 51.90% |  |  |  |
Total gross carrying value of loans |  |  | $ 919,493,000 |  |  |  |  |  |  |  | $ 919,493,000 |  |  |  | 1,289,568,000 |  |  |  | $ 919,493,000 |  | 1,289,568,000 |  | $ 919,493,000 | 1,289,568,000 |  | 1,289,568,000 | $ 919,493,000 |  |  |  |
Number of industrial facilities sold | facility |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of ground leases structured and entered into | lease |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of ground leases sold | lease |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  | $ 24,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 126,004,000 | 92,049,000 | 105,296,000 |  |  |  |  |  |
Gain on consolidation of equity method investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 67,900,000 |  |  | 67,877,000 | 0 | 0 |  |  |  |  |  |
Increase in noncontrolling interest from consolidation of equity method investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 188,300,000 |  |  | 188,279,000 |  |  |  |  |  |  |  |
Increase in redeemable noncontrolling interest from consolidation of equity method investment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11,800,000 |  |  |  |  |  |  |  |  |  |  |
Acquisitions of real estate assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19,454,000 | 6,600,000 | 38,433,000 |  |  |  |  |  |
Distributions from other investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40,804,000 | 49,672,000 | 92,482,000 |  |  |  |  |  |
Total assets |  |  | $ 2,118,045,000 |  |  |  |  |  |  |  | 2,118,045,000 |  |  |  | 2,493,798,000 |  |  |  | $ 2,118,045,000 |  | 2,493,798,000 |  | 2,118,045,000 | 2,493,798,000 |  | 2,493,798,000 | $ 2,118,045,000 |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  | $ 140,165,000 | $ 122,141,000 | $ 171,571,000 | $ 364,245,000 | 103,144,000 | $ 119,872,000 | $ 347,867,000 | $ 108,319,000 |  |  |  |  | 798,122,000 | 679,202,000 | 455,187,000 |  |  |  |  |  |
Net Lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 45,000,000 | 87,500,000 | 21,100,000 |  |  |  |  |  |
Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 81,000,000 | 4,500,000 | 75,400,000 |  |  |  |  |  |
Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lease term (in years) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 99 years |
Number of units in real estate property to be built | unit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 301 |
Leasehold improvement allowance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,200,000 |
Net Lease Venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 49,200,000 |  |  |  |  |  |  |
Net proceeds from sales of real estate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6,200,000 |  |  |  |  |  |  |
Acquisitions of real estate assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 59,000,000 |  |  |  |  |  |  |
Financing derecognized during sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 18,900,000 |  |  |  |  |  |  |
Distributions from other investments |  |  |  |  |  |  |  |  | $ 13,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 26,000,000 |  |  |  |  |  |  |
Contributions to other investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 37,700,000 |  |  |  |  |  |
Net Lease Venture | Net Lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 51.90%
Equity method investment, related party ownership percentage |  |  | 0.60% |  |  |  |  |  |  |  | 0.60% |  |  |  |  |  |  |  | 0.60% |  |  |  | 0.60% |  |  |  | 0.60% |  |  |  |
Equity method investment, related party promote fee percentage |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  |
Equity method investment, partner ownership percentage |  |  | 47.50% |  |  |  |  |  |  |  | 47.50% |  |  |  |  |  |  |  | 47.50% |  |  |  | 47.50% |  |  |  | 47.50% |  |  |  |
Total assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 658,300,000 |  |  |  |  |  | 658,300,000 |  |  | 658,300,000 |  | 658,300,000 |  |  |  |  |
Net Lease Venture II | The Properties - Livermore CA |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties | building |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lease term (in years) |  |  | 10 years |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |  |  | 10 years |  |  |  | 10 years |  |  |  | 10 years |  |  |  |
Acquisitions of real estate assets |  |  | $ 31,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of properties (sq ft) | ft² |  |  | 168,636 |  |  |  |  |  |  |  | 168,636 |  |  |  |  |  |  |  | 168,636 |  |  |  | 168,636 |  |  |  | 168,636 |  |  |  |
Percentage of acquired properties leased (percent) |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of leases | lease |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other real estate equity investments | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  | 16.00% |  |  |  |  |  |  |  | 16.00% |  |  |  |  |  |  |  | 16.00% |  |  |  | 16.00% |  |  |  | 16.00% |  |  |  |
Other real estate equity investments | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  | 95.00% |  |  |  |  |  |  |  | 95.00% |  |  |  |  |  |  |  | 95.00% |  |  |  | 95.00% |  |  |  | 95.00% |  |  |  |
Other real estate equity investments | Land |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value |  |  | $ 65,300,000 |  |  |  |  |  |  |  | $ 65,300,000 |  |  |  | 63,800,000 |  |  |  | $ 65,300,000 |  | 63,800,000 |  | $ 65,300,000 | 63,800,000 |  | 63,800,000 | $ 65,300,000 |  |  |  |
Other real estate equity investments | Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value |  |  | $ 65,600,000 |  |  |  |  |  |  |  | $ 65,600,000 |  |  |  | 38,800,000 |  |  |  | $ 65,600,000 |  | 38,800,000 |  | $ 65,600,000 | 38,800,000 |  | 38,800,000 | $ 65,600,000 |  |  |  |
Other Real Estate Equity Investment Aug 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |  |  |  |  |  |  |
Loans commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 33,000,000 |  |  |
Payments for funding under loan commitments |  |  | $ 28,400,000 |  |  |  |  |  |  |  | $ 28,400,000 |  |  |  |  |  |  |  | $ 28,400,000 |  |  |  | $ 28,400,000 |  |  |  | $ 28,400,000 |  |  |  |
Other Real Estate Equity Investment Dec 2016 | Operating Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  | 50.00% |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,100,000 | 1,900,000 | 100,000 |  |  |  |  |  |
Income from sales of real estate |  |  |  |  |  |  |  | $ 8,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,600,000 |  |  |  |  |
Distributions from other investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,600,000 |  |  |  |  |  |
Loans commitment |  |  |  |  |  |  |  | 27,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 27,000,000 |  |  |  |  |  |
Payments for funding under loan commitments |  |  | $ 27,000,000 |  |  |  |  |  |  |  | $ 27,000,000 |  |  |  | $ 25,400,000 |  |  |  | $ 27,000,000 |  | 25,400,000 |  | 27,000,000 | 25,400,000 |  | $ 25,400,000 | $ 27,000,000 |  |  |  |
Sales price of real estate held for investment |  |  |  |  |  |  |  | 36,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of other real estate, net of equity contribution |  |  |  |  |  |  |  | $ 7,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management Fees | Net Lease Venture | Other Income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,300,000 | $ 2,100,000 | $ 1,600,000 |  |  |  |  |  |
Management Fees | Net Lease Venture | Net Income Attributable To Noncontrolling Interests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |  |  |  |  |  |
Management Fees | Net Lease Venture II | Other Income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward purchase contract, purchase agreement, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 34,000,000 |
Safety, Income and Growth, Inc. | Ground Lease, Atlanta medical office |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,400,000 |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Ground Lease, 100 200 Glenridge Point |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to extend credit |  |  |  |  | $ 19,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest income |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,400,000 |  |  |  |  |  |  |  |
Number of office buildings | office_building |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leasehold First Mortgage | Great Oaks |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to extend credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 80,500,000 |
Total gross carrying value of loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 80,500,000 |
2017 Secured Financing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 227,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Net Lease Venture Nonrecourse Financing | Net Lease Venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-recourse financing |  |  |  |  |  |  |  |  | $ 29,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | 2017 Secured Financing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 227,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value of assets |  | $ 161,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity interest (percent) |  |  | 41.80% |  |  |  |  |  |  |  | 41.80% |  |  |  |  |  |  |  | 41.80% |  |  |  | 41.80% |  |  |  | 41.80% |  |  |  |
Safety, Income and Growth, Inc. | Ground Lease, Atlanta medical office |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to extend credit |  |  |  |  |  | $ 24,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument term |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commitment to extend credit, amount funded |  |  | $ 17,000,000 |  |  |  |  |  |  |  | $ 17,000,000 |  |  |  |  |  |  |  | $ 17,000,000 |  |  |  | $ 17,000,000 |  |  |  | $ 17,000,000 |  |  |  |
Safety, Income and Growth, Inc. | Private Placement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of private placement | $ 45,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | IPO |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance initial public offering | 205,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments of stock issuance costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 18,900,000 |  |  |  |  |  |  |  |  |  |
Offering costs | $ 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | 10b5-1 Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares purchased | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 133,524 |  |  |  | 2,400,000 |  |  |  |
Aggregate value of common stock purchased |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200,000 |  |  |  | $ 45,700,000 |  |  |  |
Average share price of common stock purchased (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 16.39 |  |  |  | $ 18.69 |  |  |  |
Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees waived |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,800,000 |  |  |  |  |  |  |  |  |  |  |
Reimbursement revenue waived |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 800,000 |  |  |  |  |  |  |  |  |  |  |
Common stock received as payment of management fees (in shares) | shares |  |  |  |  |  |  |  |  |  |  | 45,941 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Management Fees |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from related party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,800,000 |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Expense Reimbursement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from related party |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 700,000 |  |  |  |  |  |  |  |  |  |  |  |
Safety, Income and Growth, Inc. | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock received as payment of management fees (in shares) | shares |  |  |  |  |  |  |  |  |  | 46,020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ASU No. 2017-05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase to retained earnings upon adoption of new accounting standard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55,500,000 |  |
ASU No. 2017-05 | Other Real Estate Equity Investment Dec 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase to retained earnings upon adoption of new accounting standard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8,800,000 |  |
SAFE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  | 55,500,000 |  |  |  |  | $ 55,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ground Net Lease Business |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net proceeds from sales of real estate |  |  |  |  |  |  | $ 113,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties | property |  |  |  |  |  |  | 12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain on sale of discontinued operation |  |  |  |  |  |  | $ 123,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 178,900,000 |  |  |  |  |  |  |  |  |
Ground Net Lease Business | ASU No. 2017-05 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase to retained earnings upon adoption of new accounting standard |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 55,500,000 |  |
Acquisition Transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire equity method investments |  | 55,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition Transactions | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire interest in joint venture |  | $ 57,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition Transactions | SIGI Acquisition Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest by noncontrolling owners (percent) |  | 49.10% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition Transactions | SIGI Acquisition Inc | Safety, Income and Growth, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest by majority shareholders (percent) |  | 50.90% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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