Convertible Notes - Amortization expense, debt discount and debt issuance costs (Details) - Convertible Notes $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Amortization expense for debt discount costs
2019	$ 8,245
2020	8,864
2021	9,528
2022	10,241
2023	8,180
Total	45,058
Amortization expense for debt issuance costs
2019	849
2020	901
2021	956
2022	1,015
2023	801
Total	$ 4,522