Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Income Tax Disclosure [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		31.50%	35.00%	35.00%
Income Tax Expense (Benefit)		$ 406	$ 243	$ (279)
Tax Benefits Allocated Directly to Contributed Capital			65	83
Gross unrecognized tax benefits	$ 457	457	389	331	$ 254
Amount of unrecognized tax benefits that would affect the effective tax rate	248	248
Combined amount of accrued interest and penalties related to uncertain tax positions	18	18	14
Amount of unrecognized tax benefits for which it is reasonably possible that there will be a reduction within the next 12 months	60	60
Unremitted earnings of foreign subsidiaries	0	0	70
Deferred Tax Assets, Valuation Allowance	138	138	114
Internal Revenue Service (IRS) [Member]
Income Tax Disclosure [Line Items]
Tax credit carry forwards	8	8
Foreign Country [Member]
Income Tax Disclosure [Line Items]
Tax credit carry forwards	6	6
State [Member]
Income Tax Disclosure [Line Items]
Net operating Loss Carryforwards	639	639
Net operating loss attributable to various acquired companies	10	10
Tax credit carry forwards	$ 107	107
Including U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Disclosure [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%
Total U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Disclosure [Line Items]
Income Tax Expense (Benefit)		235
Transition tax [Member]
Income Tax Disclosure [Line Items]
Income Tax Expense (Benefit)	$ 41	192
Increase to net cash provided by Operating Activities [Member]
Income Tax Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities			$ 65	$ 86
Increase to net cash provided by Operating Activities [Member] | Provision for income taxes [Member]
Income Tax Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities		$ 43