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
	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%