Real Estate Investments and Related Intangibles - Intangible Lease Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 611	$ 688	$ 1,342	$ 2,175
Intangible lease assets:
Intangible lease assets, net	1,096,143		1,096,143		$ 1,198,506
Intangible lease liabilities:
Accumulated amortization	93,481		$ 93,481		89,905
Weighted-Average Useful Life			19 years
Intangible lease liabilities, net	152,654		$ 152,654		173,479
In-place leases and other intangible assets [Member]
Intangible lease assets:
Weighted-Average Useful Life			15 years 8 months 12 days
Intangible lease assets, net	902,319		$ 902,319		980,971
Accumulated amortization	722,414		$ 722,414		703,909
Leasing commissions [Member]
Intangible lease assets:
Weighted-Average Useful Life			10 years
Intangible lease assets, net	15,364		$ 15,364		15,660
Accumulated amortization	4,747		$ 4,747		4,048
Above-market lease assets and deferred lease incentives [Member]
Intangible lease assets:
Weighted-Average Useful Life			16 years 3 months 18 days
Intangible lease assets, net	178,460		$ 178,460		201,875
Accumulated amortization	$ 104,395		104,395		$ 105,936
Above‑ And Below-Market Leases and Deferred Lease Incentives [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization expense			1,300	2,200
In-Place Leases, Leasing Commissions and Other Lease Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization expense			$ 65,800	$ 70,100