Debt - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Disclosure [Abstract]
Contractual interest expense	$ 1,971	$ 1,971	$ 5,912	$ 6,214
Amortization of debt issuance costs	349	415	1,047	1,000
Accretion of back-end fee on debt	0	0	0	50
Accretion of debt discount	4,526	4,289	13,398	11,541
Total interest expense	$ 6,846	$ 6,675	$ 20,357	$ 18,805