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Related Party Transactions - Agreements with Carl Icahn (Details) - Nomination and Standstill Agreement - Icahn Group - shares
 12 Months Ended
Sep. 15, 2014
Dec. 31, 2017
Related Party Transaction [Line Items]    
Ownership percentage limit (no more than) 20.00%  
Net long position shares held, tranche one (in shares)   1,900,000
Net long position shares held, tranche two (in shares)   1,520,000
Net long position shares held, tranche three (in shares)   1,266,667