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

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Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
1.125% Convertible Notes
Derivative [Line Items]
Percentage of contractual interest rate	1.125%
1.125% Call Option
Derivative [Line Items]
Fixed interest rate on derivative	1.125%
1.125% Call Option \| Current assets
Derivative [Line Items]
Derivative asset	$ 522	$ 267
1.125% Conversion Option
Derivative [Line Items]
Fixed interest rate on derivative	1.125%
1.125% Conversion Option \| Current liabilities
Derivative [Line Items]
Derivative liability	$ 522	$ 267