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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Long-Term Debt
Cash interest per contractual coupon rate	$ 719	$ 719	$ 2,157	$ 2,157
Amortization of note discount	2,569	2,439	7,608	7,219
Amortization of debt issuance costs	176	159	513	463
Total interest expense related to Convertible Notes	$ 3,464	$ 3,317	$ 10,278	$ 9,839