Long-term Debt - Interest Costs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Amortization of debt discount and issuance costs			$ 19	$ 34
0.250% convertible senior notes due 2025
Debt Instrument [Line Items]
Contractual coupon interest	$ 1	$ 1	3	1
Amortization of debt discount and issuance costs	1	12	3	19
Total interest expense	$ 2	$ 13	$ 6	$ 20

Amount of amortization expense attributable to debt discount (premium) and debt issuance costs.

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