Income Taxes (Income Tax Provision) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Current provision:
Domestic (Canada)									$ 8	$ 5	$ 4
Foreign (all other countries)									139	134	98
Total current									147	139	102
Deferred provision (benefit):
Domestic (Canada)									0	0	0
Foreign (all other countries)									4	(93)	(88)
Total deferred									4	(93)	(88)
Income tax provision (benefit)	$ 41	$ 47	$ 27	$ 36	$ 18	$ 16	$ (3)	$ 15	$ 151	$ 46	$ 14