Condensed Consolidated Statements of Stockholders' Equity Statement - USD ($) shares in Thousands, $ in Millions	Total		Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (loss)
Beginning balance (in shares) at Mar. 31, 2018			306,370
Beginning balance at Mar. 31, 2018	$ 4,595		$ 3	$ 657	$ 4,062	$ (127)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect on Retained Earnings, Net of Tax | Accounting Standards Update 2014-09 [Member]					590
Cumulative Effect on Retained Earnings, Net of Tax | ASU 2018-02 [Member]					(1)
Other comprehensive income (loss) before reclassifications | Accounting Standards Update 2014-09 [Member]						22
Amounts reclassified from accumulated other comprehensive income (loss) | ASU 2018-02 [Member]						1
Cumulative Effect on Retained Earnings, Tax | Accounting Standards Update 2014-09 [Member]	612
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	371				293	78
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition	70			70
Stock Issued During Period, Shares, New Issues			1,280
Stock Issued During Period, Value, New Issues	(88)			(88)
Stock Repurchased and Retired During Period, Shares			(2,264)
Stock Repurchased and Retired During Period, Value	(300)			(300)	0
Ending balance (in shares) at Jun. 30, 2018			305,386
Ending balance at Jun. 30, 2018	5,260		$ 3	339	4,944	(26)
Beginning balance (in shares) at Mar. 31, 2018			306,370
Beginning balance at Mar. 31, 2018	4,595		$ 3	657	4,062	(127)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss) before reclassifications	69
Amounts reclassified from accumulated other comprehensive income (loss)	16
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 633
Stock Repurchased and Retired During Period, Shares	(600)
Stock Repurchased and Retired During Period, Value	$ (76)
Ending balance (in shares) at Sep. 30, 2018			303,670
Ending balance at Sep. 30, 2018	5,317		$ 3	134	5,199	(19)
Beginning balance (in shares) at Jun. 30, 2018			305,386
Beginning balance at Jun. 30, 2018	5,260		$ 3	339	4,944	(26)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss) before reclassifications	6
Amounts reclassified from accumulated other comprehensive income (loss)	1
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	262				255	7
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition	66			66
Stock Issued During Period, Shares, New Issues			618
Stock Issued During Period, Value, New Issues	28			28
Stock Repurchased and Retired During Period, Shares			(2,334)
Stock Repurchased and Retired During Period, Value	(299)			(299)	0
Ending balance (in shares) at Sep. 30, 2018			303,670
Ending balance at Sep. 30, 2018	$ 5,317		$ 3	134	5,199	(19)
Beginning balance (in shares) at Mar. 31, 2019	298,000		298,107
Beginning balance at Mar. 31, 2019	$ 5,331	[1]	$ 3	0	5,358	(30)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	1,434				1,421	13
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition	73			73
Stock Issued During Period, Shares, New Issues			985
Stock Issued During Period, Value, New Issues	(48)			(48)
Stock Repurchased and Retired During Period, Shares			(3,205)
Stock Repurchased and Retired During Period, Value	(305)			(25)	(280)
Ending balance (in shares) at Jun. 30, 2019			295,887
Ending balance at Jun. 30, 2019	$ 6,485		$ 3	0	6,499	(17)
Beginning balance (in shares) at Mar. 31, 2019	298,000		298,107
Beginning balance at Mar. 31, 2019	$ 5,331	[1]	$ 3	0	5,358	(30)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss) before reclassifications	55
Amounts reclassified from accumulated other comprehensive income (loss)	(35)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 2,295
Ending balance (in shares) at Sep. 30, 2019	293,000		293,218
Ending balance at Sep. 30, 2019	$ 7,158		$ 3	0	7,165	(10)
Beginning balance (in shares) at Jun. 30, 2019			295,887
Beginning balance at Jun. 30, 2019	6,485		$ 3	0	6,499	(17)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive income (loss) before reclassifications	32
Amounts reclassified from accumulated other comprehensive income (loss)	(25)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	861				854	7
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition	92			92
Stock Issued During Period, Shares, New Issues			584
Stock Issued During Period, Value, New Issues	26			26
Stock Repurchased and Retired During Period, Shares			(3,253)
Stock Repurchased and Retired During Period, Value	$ (306)			(118)	(188)
Ending balance (in shares) at Sep. 30, 2019	293,000		293,218
Ending balance at Sep. 30, 2019	$ 7,158		$ 3	$ 0	$ 7,165	$ (10)

[1] Derived from audited Consolidated Financial Statements.