Income Tax Considerations (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Required distribution percentage to maintain REIT status (at least)	90.00%
Fixed assets book value in excess of (less than) tax basis	$ 193.4	$ 268.7
Current tax obligation	$ 1.6	$ 1.0