Share-based Compensation Expense - Narrative (Details) $ in Millions |
12 Months Ended |
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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 |