Unit Capital - Additional Information (Detail) - USD ($)		12 Months Ended
	Jun. 11, 2014	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Capital Unit [Line Items]
Units Issued During the Period		9,539,710	14,529,254
Held Units For Minimum year	1 year
Unit Repurchase Price Description		price equal to the then current offering price less the sales fees associated with that class of units.
Net assets value per unit		$ 8.466	$ 8.469
Repurchase of Unit Total value		$ 8,699,994	$ 10,362,875
Maximum [Member]
Capital Unit [Line Items]
Percentage of Total	5.00%
Amended Unit Repurchase Program [Member]
Capital Unit [Line Items]
Unit Repurchase Price Description		price equal to the greater of $9.025 or the estimated net asset per unit for each class of units, as most recently disclosed by us in a public filing with the SEC.
Unit redeemed per share							$ 9.025
Further Amended Unit Repurchase Program [Member]
Capital Unit [Line Items]
Unit Repurchase Price Description		price equal to the most recently determined net asset value per unit for each class of units, as most recently disclosed by us in a public filing with the SEC at the time of redemption. Redemptions for the first quarter of 2018 were redeemed at the price equal to $8.507 for Class A units, Class C units, Class I units and Class Y unit, which was the net asset value per unit of each class as of September 30, 2017, the most recently disclosed net asset value at the time of redemption. Redemptions for the second quarter of 2018 will be redeemed at a price equal to $8.466 per Class A unit, Class C units, Class I unit and Class Y units, which was the net asset value per unit of each class as of December 31, 2017.
Distribution Reinvestment Plan [Member]
Capital Unit [Line Items]
Units Issued During the Period		1,249,131
Units Issued During the Period, value		$ 11,267,412
Class C Units [Member]
Capital Unit [Line Items]
Distribution and dealer manager fee payable		$ 1,895,000
Estimated net assets value per unit		$ 9.025
Percentage of distribution and dealer manager fee per annum		0.80%
Distribution and dealer manager fee payable, discount rate		4.00%
Units Issued During the Period		1,672,115	5,739,329
Net assets value per unit		$ 8.466	$ 8.534			$ 8.507		$ 8.267
Repurchase of Aggregate units		64,757	8,943
Repurchase Price Per unit		$ 9.025	$ 9.025
Class C Units [Member] | Further Amended Unit Repurchase Program [Member] | Scenario, Forecast [Member]
Capital Unit [Line Items]
Unit redeemed per share				$ 8.466
Class C Units [Member] | Further Amended Unit Repurchase Program [Member] | Subsequent Event [Member]
Capital Unit [Line Items]
Unit redeemed per share					$ 8.507
Class I Units [Member]
Capital Unit [Line Items]
Distribution and dealer manager fee payable		$ 1,895,000
Estimated net assets value per unit		$ 9.025
Percentage of distribution and dealer manager fee per annum		0.50%
Distribution and dealer manager fee payable, discount rate		4.00%
Units Issued During the Period		3,526,396	2,541,539
Net assets value per unit		$ 8.466	$ 8.534			8.507		8.529
Repurchase of Aggregate units		495,792	573,750
Repurchase Price Per unit		$ 9.025	$ 9.025
Class I Units [Member] | Further Amended Unit Repurchase Program [Member] | Scenario, Forecast [Member]
Capital Unit [Line Items]
Unit redeemed per share				8.466
Class I Units [Member] | Further Amended Unit Repurchase Program [Member] | Subsequent Event [Member]
Capital Unit [Line Items]
Unit redeemed per share					8.507
Class A Units [Member]
Capital Unit [Line Items]
Units Issued During the Period		3,251,521	6,248,387
Net assets value per unit		$ 8.466	$ 8.534			8.507		$ 8.529
Repurchase of Aggregate units		403,439	565,549
Repurchase Price Per unit		$ 9.025	$ 9.025
Class A Units [Member] | Further Amended Unit Repurchase Program [Member] | Scenario, Forecast [Member]
Capital Unit [Line Items]
Unit redeemed per share				8.466
Class A Units [Member] | Further Amended Unit Repurchase Program [Member] | Subsequent Event [Member]
Capital Unit [Line Items]
Unit redeemed per share					8.507
Class Y Units [Member]
Capital Unit [Line Items]
Units Issued During the Period		1,089,678
Net assets value per unit		$ 8.466	$ 8.534			$ 8.507
Class Y Units [Member] | Further Amended Unit Repurchase Program [Member] | Scenario, Forecast [Member]
Capital Unit [Line Items]
Unit redeemed per share				$ 8.466
Class Y Units [Member] | Further Amended Unit Repurchase Program [Member] | Subsequent Event [Member]
Capital Unit [Line Items]
Unit redeemed per share					$ 8.507