Consolidated Statements Of Stockholders' Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock	Additional paid-in capital	Retained earnings (Accumulated deficit)	Accumulated other comprehensive income (loss)	Repurchase program, total
Balance at Mar. 31, 2015	$ 3,036	$ 3	$ 2,127	$ 904	$ 2
Balance (in shares) at Mar. 31, 2015		309,681
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	1,138			1,156	(18)
Issuance of common stock	(49)		(49)
Issuance of common stock (in shares)		6,645
Reclassification of equity component of convertible notes	29		29
Settlement of convertible notes	(1)		(1)
Settlement of Convertible Notes		7,823
Exercise of convertible note hedge		(7,823)
Repurchase and retirement of common stock	(1,018)		(1,018)			$ (1,018)
Stock repurchased and retired during period, Shares		(15,724)				15,700
Stock-based compensation	178		178
Tax benefit from stock-based compensation	83		83
Balance at Mar. 31, 2016	3,396	$ 3	1,349	2,060	(16)
Balance (in shares) at Mar. 31, 2016		300,602
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	964			967	(3)
Issuance of common stock	(55)		(55)
Issuance of common stock (in shares)		4,626
Reclassification of equity component of convertible notes	2		2
Settlement of convertible notes	0
Settlement of Convertible Notes		(2,917)
Exercise of convertible note hedge		2,917
Repurchase and retirement of common stock	(508)		(508)			$ (508)
Stock repurchased and retired during period, Shares		(6,506)				6,500
Settlement of warrants		9,645
Stock-based compensation	196		196
Tax benefit from stock-based compensation	65		65
Balance at Mar. 31, 2017	$ 4,060	$ 3	1,049	3,027	(19)
Balance (in shares) at Mar. 31, 2017	308,000	308,367
Cumulative Effect on Retained Earnings, before Tax			9
Cumulative Effect on Retained Earnings, Net of Tax				(8)
Cumulative Effect on Retained Earnings, Tax	$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	935			1,043	(108)
Issuance of common stock	(42)		(42)
Issuance of common stock (in shares)		3,332
Repurchase and retirement of common stock	(601)		(601)			$ (601)
Stock repurchased and retired during period, Shares		(5,329)				5,300
Stock-based compensation	242		242
Balance at Mar. 31, 2018	$ 4,595	$ 3	$ 657	$ 4,062	$ (127)
Balance (in shares) at Mar. 31, 2018	306,000	306,370