Long-Term Debt - Convertible Notes Interest Expense (Details) - Convertible Senior Notes 1.00 percent due 2020 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-Term Debt
Cash interest per contractual coupon rate	$ 2,875	$ 2,875	$ 2,875
Amortization of note discount	10,210	9,690	9,194
Amortization of debt issuance costs	695	624	559
Total interest expense related to Convertible Notes	$ 13,780	$ 13,189	$ 12,628