Real Estate (Dispositions) (Details)		1 Months Ended		3 Months Ended				6 Months Ended				12 Months Ended
	Apr. 14, 2017 USD ($)	Jun. 30, 2018 USD ($)	Apr. 30, 2017 USD ($) lease property	Jun. 30, 2018 USD ($)		Jun. 30, 2017 USD ($)		Jun. 30, 2018 USD ($)		Jun. 30, 2017 USD ($)		Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)	Mar. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sales of real estate		$ 36,100,000						$ 238,834,000		$ 154,291,000
Payments to acquire equity method investments								53,012,000		139,139,000
Income from sales of real estate		24,500,000		$ 56,895,000	[1]	$ 844,000	[1]	73,943,000	[1]	8,954,000	[1]
Retained deficit		$ (2,325,291,000)		$ (2,325,291,000)				$ (2,325,291,000)				$ (2,470,564,000)
Safety, Income and Growth, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Carrying value of properties			$ 161,100,000
Income from sales of real estate			$ 123,400,000
Safety, Income and Growth, Inc. | 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 ground net leases | 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		$ 113,000,000
Payments to acquire equity method investments	$ 55,500,000		$ 55,500,000
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				$ 227,000,000.0				$ 227,000,000.0
Accounting Standards Update 2017-05
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Retained deficit													$ (2,394,695,000)
Accounting Standards Update 2017-05 | New Accounting Pronouncement, Early Adoption, Effect
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Retained deficit													$ 55,500,000
Accounting Standards Update 2017-05 | New Accounting Pronouncement, Early Adoption, Effect | Ground Net Lease Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of business												$ 178,900,000

[1] Income from sales of real estate represents gains from sales of real estate that do not qualify as discontinued operations