Restructuring and Impairment (Restructuring Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Restructuring Cost and Reserve [Line Items]
Impairment charges	$ 2	$ 25	$ 7
Restructuring Reserve	24	27	32		$ 47
Restructuring charges	8	45	35
Restructuring payments	(13)	(22)	(32)
Corporate Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring payments	(1)	(10)	0
Corporate Segment [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	1
Restructuring payments	(1)	(10)	0
Corporate Segment [Member] | Corporate Restructuring Program [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	35	0	$ 0
Corporate Segment [Member] | Corporate Restructuring Program [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	12	0	0
Corporate Segment [Member] | Corporate Restructuring Program [Member] | Fixed Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	21	0	0
Corporate Segment [Member] | Corporate Restructuring Program [Member] | Period Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	2	0	0
North America [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1	1	1	38
Restructuring payments	0	(1)	(3)
North America [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring payments	0	0	(2)
North America [Member] | Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring payments	0	(1)	(1)
North America [Member] | Saguenay Plant Closure [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	0	0	5
North America [Member] | Saguenay Plant Closure [Member] | Fixed Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	0	0	28
North America [Member] | Saguenay Plant Closure [Member] | Other Exit Related Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1	0	1	1
North America [Member] | Saguenay Plant Closure [Member] | Period Expenses [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	1	0	4
Europe [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2	4	3	43
Restructuring payments	(4)	(6)	(12)
Europe [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	2
Restructuring payments	(4)	(6)	(12)
Europe [Member] | Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring payments	0	0	0
Europe [Member] | Business Optimization [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	0	3	42
Europe [Member] | Business Optimization [Member] | Pension Curtailment Loss [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	0	0	1
Europe [Member] | Corporate Optimization [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2	4	0	0
South America [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	19
Restructuring charges	5	3	31	48
Restructuring payments	(7)	(5)	(16)
South America [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring payments	(2)	(2)	(12)
South America [Member] | Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring payments	(5)	(3)	(4)
South America [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve	7
South America [Member] | Ouro Preto Smelter Closures [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1	2	14	5
South America [Member] | Ouro Preto Smelter Closures [Member] | Fixed Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	0	5	1
South America [Member] | Ouro Preto Smelter Closures [Member] | Environmental Restoration Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	(1)	6	16
South America [Member] | Ouro Preto Smelter Closures [Member] | Contract Termination [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2	2	5	6
South America [Member] | Other South America Restructuring Programs [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2	0	0	0
South America [Member] | Other Past Restructuring Programs [Member] | Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	0	0	7
South America [Member] | Other Past Restructuring Programs [Member] | Fixed Asset Impairment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0	0	0	7
South America [Member] | Other Past Restructuring Programs [Member] | Other Exit Related Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0	$ 0	$ 1	$ 6