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Share-based Compensation Expense - Narrative (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Unrecognized compensation cost related to restricted stock units and ESPP shares issued to date $ 394
Weighted-average period of unrecognized compensation cost related to restricted stock and ESPP shares issued to date 2 years 8 months