Real Estate Investments and Related Intangibles - Unconsolidated Joint Ventures Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) property joint_venture	Dec. 31, 2016 USD ($) property joint_venture	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	$ 42,784,000	$ 46,077,000
Net (loss) income	$ 2,726,000	(415,000)	$ 2,361,000
Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Number of properties owned | property	472
Aggregate balance outstanding	$ 2,071,038,000	2,629,949,000
Unconsolidated Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate equity investments	39,500,000	41,300,000
Net (loss) income	3,300,000	$ 900,000	$ 2,300,000
Unconsolidated Joint Venture [Member] | Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate balance outstanding	20,400,000
Unconsolidated Joint Venture [Member] | Mortgages [Member] | Recourse debt [Member]
Schedule of Equity Method Investments [Line Items]
Aggregate balance outstanding	$ 0
Unconsolidated Properties [Member]
Schedule of Equity Method Investments [Line Items]
Number of unconsolidated joint ventures | joint_venture	2	2
Number of properties owned | property	1	1