Capital Stock (Common Shares Reserved for Future Issuance) (Details) - shares shares in Millions	Sep. 30, 2018	Apr. 15, 2018	Mar. 31, 2018
Common Shares Reserved For Future Issuance [Line Items]
Shares reserved for future issuance (in shares)	44.2		46.7
Stock Options and Equity-Settled SARs
Common Shares Reserved For Future Issuance [Line Items]
Shares reserved for future issuance (in shares)	33.8		32.1
Restricted Stock and Restricted Share Units - Unvested
Common Shares Reserved For Future Issuance [Line Items]
Shares reserved for future issuance (in shares)	2.3		2.2
Common shares available for future issuance under the 2017 Plan
Common Shares Reserved For Future Issuance [Line Items]
Shares reserved for future issuance (in shares)	8.1		10.3
Convertible Senior Subordinated Notes | April 2013 Notes
Common Shares Reserved For Future Issuance [Line Items]
Shares reserved for future issuance (in shares)	0.0		2.1
April 2013 Notes | Convertible Senior Subordinated Notes
Common Shares Reserved For Future Issuance [Line Items]
Coupon rate	1.25%	1.25%