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Convertible Senior Notes (Summary of Interest Expense for Contractual Interest and Accretion of Debt Discount) (Details) - Convertible Senior Notes [Member] - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
2028 Convertible Senior Notes [Member]        
Debt Instrument [Line Items]        
Contractual coupon interest expense     $ 1,467  
Amortization of debt discount     1,921  
Amortization of debt issuance costs     36  
Total interest expense related to the Notes     3,424  
2019 Convertible Senior Notes [Member]        
Debt Instrument [Line Items]        
Contractual coupon interest expense $ 1,361 $ 1,856 5,074 $ 5,569
Amortization of debt discount 1,891 2,440 6,907 7,188
Amortization of debt issuance costs 307 381 1,104 1,106
Total interest expense related to the Notes $ 3,559 $ 4,677 $ 13,085 $ 13,863