Long-Term Debt - Convertible Notes Interest Expense (Details) - Convertible Senior Notes 1.00 percent due 2020 - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Long-Term Debt
Cash interest per contractual coupon rate	$ 719	$ 719	$ 1,438	$ 1,438
Amortization of note discount	2,536	2,406	5,039	4,780
Amortization of debt issuance costs	171	154	337	304
Total interest expense related to Convertible Notes	$ 3,426	$ 3,279	$ 6,814	$ 6,522