Contractual interest rate for funds borrowed, under the debt agreement.

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Derivatives (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
1.125% Convertible Notes
Derivative [Line Items]
Percentage of contractual interest rate on Notes	1.125%
1.125% Call Option
Derivative [Line Items]
Percentage of contractual interest rate on Call Option	1.125%
1.125% Conversion Option derivative liability
Derivative [Line Items]
Percentage of contractual interest rate on Call Option	1.125%
Current assets: Derivative asset \| 1.125% Call Option
Derivative [Line Items]
Derivative assets	$ 585	$ 522
Current liabilities: Derivative liability \| 1.125% Conversion Option derivative liability
Derivative [Line Items]
Derivative liabilities	$ 585	$ 522