Effects on net income of amounts reclassified from accumulated other comprehensive income (loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	$ (5)	$ (5)	$ (11)	$ (8)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	(1)	(1)	(1)	(1)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member] | Interest Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	(1)	(1)	(1)	(1)
Unrealized Gains (Losses) on Derivative Instruments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	(4)	(4)	(10)	(7)
Unrealized Gains (Losses) on Derivative Instruments [Member] | Sales Revenue, Net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	(4)	(6)	(9)	(14)
Unrealized Gains (Losses) on Derivative Instruments [Member] | Research And Development [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income (loss)	$ 0	$ 2	$ (1)	$ 7