Summary of Significant Accounting Policies (Additional Information 2) (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) segment subsidiary	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Apr. 05, 2017 USD ($)	Dec. 31, 2014 subsidiary
Summary Of Significant Accounting Policies [Line Items]
Aggregate allowance for doubtful accounts	$ 10,800		$ 11,500
Capital lease obligations	5,700		7,800
Lease financing obligations	700		900
Net book value of assets associated with leases	$ 12,600		$ 14,700
Number of operating segments | segment	1
Number of reportable segments | segment	1
Number of subsidiaries taxed as taxable REIT | subsidiary	2
Number of REIT subsidiaries' deferred tax assets due to U.S. NOL carryforwards | subsidiary					1
Effective tax rate	23.10%	39.70%
Statutory rate	21.00%		35.00%
Increase in fair value of interest rate swaps	$ 5,982
Other Assets.
Summary Of Significant Accounting Policies [Line Items]
Fair value	$ 7,400
Interest Rate Swap
Summary Of Significant Accounting Policies [Line Items]
Derivative instruments, notional amount				$ 400,000
Increase in fair value of interest rate swaps		$ 0
Interest Rate Swap | Term Loan
Summary Of Significant Accounting Policies [Line Items]
Debt Instrument Face Amount				400,000
Interest Rate Swap | Term Loan Maturing December 17,2021
Summary Of Significant Accounting Policies [Line Items]
Derivative instruments, notional amount				200,000
Interest Rate Swap | Term Loan Maturing April 27, 2022
Summary Of Significant Accounting Policies [Line Items]
Derivative instruments, notional amount				$ 200,000
Accounts Receivable | Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of total accounts receivable	10.00%
Customer One | Rental Revenue
Summary Of Significant Accounting Policies [Line Items]
Percentage of total revenue	11.80%
No Other Customers | Rental Revenue | Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of total revenue	5.00%
Five Customers | Accounts Receivable
Summary Of Significant Accounting Policies [Line Items]
Percentage of total accounts receivable	36.00%
Five Customers | Accounts Receivable | Minimum
Summary Of Significant Accounting Policies [Line Items]
Percentage of total accounts receivable	5.00%