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Consolidated Statements of Stockholder's Equity (Parenthetical) - shares
shares in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stockholders' Equity [Roll Forward]      
Employee stock options (in shares) 1.2 1.1 0.7
Acquisition of equity method investment with treasury stock (in shares) 0.2