Real Estate Investments (Narrative) (Details) $ in Millions	Sep. 30, 2017 USD ($) ft²	Jun. 15, 2016 USD ($) ft²
Real Estate [Line Items]
Area of real estate | ft²	1,100,000
Aggregate purchase price of real estate | $	$ 686.1
1140 Avenue of the Americas
Real Estate [Line Items]
Area of real estate | ft²		249,703
Aggregate purchase price of real estate | $		$ 180.0