Significant Accounting Policies - Revenue Recognition (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) category deliverable
Revenue recognition [Line items]
Maximum period of customer obligations (in years)	1 year
Bad debt expense | $	$ 37.5
Number of deliverables per unit of accounting for deferral of all revenue, greater than | deliverable	1
Number of categories for each segment in which total revenues are reported | category	2
Unconsolidated real estate affiliates
Investments [Line Items]
Ownership percentage	50.00%