Stock-Based Compensation and Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Numerator for basic and diluted earnings per share:
Net income	$ 280	$ 145
Denominator:
Denominator for basic earnings per share – weighted average common shares (in shares)	163.4	130.4
Effect of dilutive securities:
Stock options (in shares)	1.3	1.0
Performance shares, restricted stock and restricted stock units (in shares)	0.7	0.5
Dilutive potential common shares (in shares)	2.0	1.5
Denominator for diluted earnings per share – adjusted weighted average shares and assumed conversion (in shares)	165.4	131.9
Earnings per share:
Basic (in dollars per share)	$ 1.71	$ 1.11
Diluted (in dollars per share)	$ 1.69	$ 1.10
Stock Options
Earnings per share:
Stock options excluded from the average outstanding diluted shares calculation (in shares)	0.0	0.7