Related Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Due from affiliates	$ 33,401,000		$ 33,401,000		$ 35,172,000
Hunt Utility Services, LLC
Related Party Transaction [Line Items]
Payment for management fee			10,300,000	$ 14,100,000
Prepaid management fee	0		0		0
Accrued management fee	$ 0		$ 0		0	$ 3,500,000
Management agreement expiration date			Dec. 31, 2019
Agreement successive renewal terms			5 years
Management fee, description			The base fee for each 12 month period beginning each April 1 will equal 1.50% of our total equity as of December 31 of the immediately preceding year, subject to a $30.0 million cap
Investment management fee equity multiplier	1.50%		1.50%
Management fee cap	$ 30,000,000		$ 30,000,000
Hunt Utility Services, LLC | License and Maintenence Fees
Related Party Transaction [Line Items]
Reimbursement of annual software license and maintenance fees and other expenses			100,000	200,000
Sharyland
Related Party Transaction [Line Items]
Lease revenue from related party	48,900,000	$ 51,600,000	142,400,000	131,700,000
Deferred rent liability	14,600,000		14,600,000		14,700,000
Payments to acquire plant, and equipment			48,300,000	$ 151,200,000
Accounts payable and accrued liabilities	5,000,000		5,000,000		2,100,000
Due from affiliates	$ 33,400,000		$ 33,400,000		$ 35,200,000