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Net (Loss) Income Per Share (Narrative) (Details) - Convertible Senior Notes [Member] - USD ($)
Sep. 30, 2018
Aug. 31, 2018
Aug. 24, 2018
Dec. 31, 2017
Jun. 30, 2014
2019 Convertible Senior Notes [Member]          
Debt Instrument [Line Items]          
Convertible senior notes, issued $ 66,138,000     $ 329,972,000 $ 330,000,000
Convertible senior notes, interest rate         2.25%
2028 Convertible Senior Notes [Member]          
Debt Instrument [Line Items]          
Convertible senior notes, issued $ 880,000,000 $ 880,000,000      
Convertible senior notes, interest rate   2.00% 2.00%