Postretirement Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Pension Benefit Plans [Member]
Components of net periodic benefit cost for postretirement benefit plans
Service cost	$ 11	$ 13
Interest cost	17	16
Expected return on assets	(17)	(16)
Amortization-losses	6	5
Amortization — prior service credit, net	(1)	0
Termination benefits / curtailments	1	0
Net periodic benefit cost	17	18
Other Benefit Plans [member]
Components of net periodic benefit cost for postretirement benefit plans
Service cost	1	3
Interest cost	1	2
Expected return on assets	0	0
Amortization-losses	2	1
Amortization — prior service credit, net	(10)	0
Termination benefits / curtailments	(2)	0
Net periodic benefit cost	$ (8)	$ 6