Summary of Significant Accounting Policies (Additional Information 2) (Details) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) segment subsidiary	Jun. 30, 2017	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Aggregate allowance for doubtful accounts	$ 4.4		$ 11.5
Capital lease obligations	3.9		7.8
Lease financing obligations	0.5		0.9
Net book value of assets associated with leases	$ 10.9		$ 14.7
Number of operating segments | segment	1
Number of reportable segments | segment	1
Number of subsidiaries taxed as taxable REIT | subsidiary	2
Effective tax rate	10.80%	39.10%
Statutory rate	21.00%		35.00%
Customer One | Rental Revenue
Summary Of Significant Accounting Policies [Line Items]
Percentage of total revenue	12.10%
No Other Customers | Rental Revenue | Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of total revenue	5.00%
Four Customers [Member] | Accounts Receivable
Summary Of Significant Accounting Policies [Line Items]
Percentage of total accounts receivable	31.00%
Four Customers [Member] | Accounts Receivable | Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of total accounts receivable	5.00%
One Customer | Accounts Receivable | Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of total accounts receivable	10.00%