Convertible Notes and Credit Facilities - Interest (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Contractual interest expense	$ 5,130	$ 5,130	$ 5,130
Amortization of Debt Discount	5,408	4,898	4,430
Amortization of debt issuance costs	652	591	534
Total interest expense recognized	$ 11,190	$ 10,619	$ 10,094

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Represents the portion of interest incurred in the period on debt arrangements that was charged against earnings, excluding amortization of debt discount (premium) and financing costs.

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