Commitments and Contingencies (Textual) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Future obligations under leases	$ 46.8
Outstanding letters of credit and performance bonds	3.5
Lease operating expenses	$ 3.3	$ 2.4	$ 2.0
Other Commitments [Line Items]
Weighted average remaining term (in years)	2 years
Ground Leases
Other Commitments [Line Items]
Weighted average remaining term (in years)	77 years
Ground and Operating Lease Commitments
Other Commitments [Line Items]
Future lease commitments	$ 208.3