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Real Estate (Disposition of Ground Lease Business) (Details)		1 Months Ended		9 Months Ended	12 Months Ended
	Apr. 14, 2017 USD ($)	Jun. 30, 2018 USD ($)	Apr. 30, 2017 USD ($) lease property	Jan. 02, 2018 USD ($)	Dec. 31, 2018 USD ($) lease_asset	Dec. 31, 2017 USD ($) lease_asset	Dec. 31, 2016 USD ($) lease_asset property	Jan. 01, 2018 USD ($)	Mar. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties \| property							2
Number of net lease assets \| lease_asset					2	1	1
Net proceeds from sales of real estate		$ 36,100,000			$ 411,786,000	$ 314,013,000	$ 435,560,000
Payments to acquire equity method investments					94,578,000	224,219,000	58,197,000
Gain on sale of discontinued operation					$ 0	$ 123,418,000	$ 0
Increase to retained earnings upon adoption of new accounting standard								$ 75,869,000
ASU No. 2017-05
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Increase to retained earnings upon adoption of new accounting standard								55,500,000
SAFE
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to acquire equity method investments	$ 55,500,000		$ 55,500,000
Safety, Income and Growth, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Carrying value of lease assets			$ 161,100,000
2017 Secured Financing
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt instrument amount									$ 227,000,000.0
Secured Debt \| 2017 Secured Financing
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Debt instrument amount									$ 227,000,000.0
Ground Net Lease Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties \| property			12
Number of net lease assets \| lease			7
Number of master leases \| lease			1
Number of properties covered under master lease agreement \| property			5
Net proceeds from sales of real estate			$ 113,000,000
Gain on sale of discontinued operation			$ 123,400,000	$ 178,900,000
Ground Net Lease Business \| ASU No. 2017-05
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Increase to retained earnings upon adoption of new accounting standard								$ 55,500,000