Fair Value Measurements - Schedule of Fair Value of Long-term Borrowings (Parenthetical) (Detail)			6 Months Ended	12 Months Ended
	Jan. 17, 2017	Jul. 18, 2013	Jun. 30, 2018	Dec. 31, 2017
5.0% Exchangeable Senior Notes due March 2021 [Member] | Exchangeable Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate	5.00%		5.00%	5.00%
Due date	Mar. 01, 2021		Mar. 01, 2021	Mar. 01, 2021
6.625% Senior Notes Due 2021 [Member] | Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate		6.625%	6.625%	6.625%
Due date		Jul. 15, 2021	Jul. 15, 2021	Jul. 15, 2021