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Earnings Per Share - Narrative (Details) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of options excluded from the computation of diluted earnings per share	0.3	0.3	0.2	0.3
Share capital [Member]
Earnings Per Share [Abstract]
Number of shares outstanding	142.6	145.0	142.6	145.0	144.9	146.3