Debt - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Debt Instrument [Line Items]
Total interest expense	$ 2,813	$ 2,665	$ 4,151	$ 5,109
Senior Notes
Debt Instrument [Line Items]
Contractual interest expense	819	629	957	1,258
Amortization of debt discount	0	1,405	0	2,796
Amortization of debt issuance costs	1,286	187	2,075	373
Total interest expense	2,105	2,221	3,032	4,427
1.75% Convertible Senior Notes due 2023 | Senior Notes
Debt Instrument [Line Items]
Contractual interest expense	104	629	208	1,258
Amortization of debt discount	0	1,405	0	2,796
Amortization of debt issuance costs	39	187	110	373
Total interest expense	143	2,221	318	4,427
0% Convertible Senior Notes due 2025 | Senior Notes
Debt Instrument [Line Items]
Contractual interest expense	0	0	0	0
Amortization of debt discount	0	0	0	0
Amortization of debt issuance costs	690	0	1,380	0
Total interest expense	690	0	1,380	0
0.5% Convertible Senior Notes Due 2027 | Senior Notes
Debt Instrument [Line Items]
Contractual interest expense	715	0	749	0
Amortization of debt discount	0	0	0	0
Amortization of debt issuance costs	557	0	585	0
Total interest expense	$ 1,272	$ 0	$ 1,334	$ 0