Investment in and Advances to Non-Consolidated Entities - Schedule of Investments in and Non-consolidated Entities (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($) a property partnership term	Dec. 31, 2018 USD ($) a building partnership term	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 66,183	$ 66,183	$ 17,476
Proceeds from sale of property		898,514	223,853	$ 370,038
Payments to acquire interest in joint venture		0	7,951	$ 0
Real estate, at cost	3,090,134	3,090,134	3,936,459
NNN Office Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Non-recourse debt	466,200	466,200
Mortgages [Member] | NNN Office Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Non-recourse debt	$ 362,800	$ 362,800
Long-term debt, term	3 years	3 years
Number of additional extensions available | term	2	2
Term of additional extensions		1 year
Term of contract		2 years
Mortgages [Member] | NNN Office Joint Venture [Member] | London Interbank Offered Rate (LIBOR) [Member]
Schedule of Equity Method Investments [Line Items]
Basis spread on variable rate		2.00%
Basis spread rate increase per extension		0.15%
Cap interest rate	4.00%	4.00%
Office Building [Member]
Schedule of Equity Method Investments [Line Items]
Number of properties sold | property	6
NNN Office Joint Venture Properties [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments, assets	$ 757,811	$ 757,811
Equity method investments, liabilities	$ 492,091	$ 492,091
NNN Office Joint Venture Properties [Member] | Office Building [Member] | Transferred Property [Member]
Schedule of Equity Method Investments [Line Items]
Number of properties sold | building		21
Proceeds from sale of property		$ 725,800
Minimum [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage	15.00%	15.00%
Maximum [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage	25.00%	25.00%
NNN Office Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage	20.00%	20.00%
Equity method investments	$ 53,144	$ 53,144	0
NNN Office Joint Venture [Member] | 2 of 21 Transferred Properties [Member]
Schedule of Equity Method Investments [Line Items]
Contribution of property		45,653
Payments to acquire interest in joint venture		8,053
Gain (loss) on disposition of assets		14,645
Transfer mortgage payable		$ 103,400
Etna Park 70 [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage	90.00%	90.00%
Equity method investments	$ 4,774	$ 4,774	$ 5,831
Etna GL 66 LLC [Member]
Schedule of Equity Method Investments [Line Items]
Area of land (in acres) | a	57	57	151
Real estate, at cost	$ 3,008	$ 3,008
Other Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 8,265	$ 8,265	$ 11,645
Number of joint venture interests | partnership	2	2
Proceeds from collection of loans receivable			$ 49,085