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Organization - Use of the Proceeds from the IP (Parenthetical) (Detail)
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Senior Notes [Member]      
Consolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]      
Interest rate on debt 11.00% 11.00% 11.00%