Notes Payable - 2.25% Convertible Senior Notes due 2022 - Total Interest Expense (Details) - 2.25% Convertible Senior Notes due 2022 - Convertible Senior Notes - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Interest Expense
Contractual interest expense	$ 1,780	$ 1,889	$ 5,557	$ 5,665
Amortization of debt issuance costs	301	249	856	712
Amortization of debt discount	4,460	3,900	12,699	11,450
Total interest expense	$ 6,541	$ 6,038	$ 19,112	$ 17,827