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Share-Based Payments - Data Related to All Total Shareholder Return Units (Details) - Total Shareholder Return Units (TSRUs) [Member] - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average grant-date fair value per stock option (in dollars per share) $ 6.23 $ 5.83 $ 6.66
Total compensation cost related to nonvested awards not yet recognized, pre-tax $ 232 $ 164 $ 29
Weighted-average period over which nonvested award cost is expected to be recognized (years) 1 year 8 months 12 days 1 year 10 months 24 days 1 year 9 months 18 days