Long-Term Debt - Convertible Notes Interest Expense (Details) - Convertible Senior Notes 1.00 percent due 2020 - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Long-Term Debt
Cash interest per contractual coupon rate	$ 719	$ 719	$ 2,157	$ 2,157
Amortization of note discount	2,708	2,569	8,018	7,608
Amortization of debt issuance costs	196	176	572	513
Total interest expense related to Convertible Notes	$ 3,623	$ 3,464	$ 10,747	$ 10,278

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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