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Computation of Basic/Diluted Earnings Per Common Share - Narrative (Details) - shares
shares in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Abstract]      
Participating common stock weighted-average shares, unvested restricted stock rights 0 3 8
Shares not included in computation of diluted EPS due to unmet performance measures 7 8 3
Options      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Antidilutive securities excluded from computation of diluted earnings per share (in shares) 1 5 1