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Business Combinations - Acquisition of CoverMyMeds (Details) - USD ($) $ in Millions			1 Months Ended	12 Months Ended
	Mar. 31, 2018	Apr. 03, 2017	May 24, 2018	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2019
Business Acquisition [Line Items]
Purchase consideration paid in cash, net of cash acquired				$ 2,893	$ 4,237	$ 40
Goodwill	$ 10,924			10,924	$ 10,586	$ 9,786
CMM
Business Acquisition [Line Items]
Purchase consideration paid in cash, net of cash acquired		$ 1,300
Fair value of assets acquired (excluding goodwill and intangibles)	53			53
Fair value of liabilities assumed	8			8
Goodwill	870			870
Expected | CMM
Business Acquisition [Line Items]
Contingent consideration							$ 160
Investment Assets at Fair Value | CMM
Business Acquisition [Line Items]
Contingent consideration	124	$ 113		124
Customer relationships | CMM
Business Acquisition [Line Items]
Acquired identifiable intangibles	$ 487			$ 487
Weighted average life	17 years
Subsequent Event | Investment Assets at Fair Value | CMM
Business Acquisition [Line Items]
Cash apayment for contingent consideration earned			$ 68

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Business Combination, Recognized Identifiable Assets Acquired, Assets Excluding Goodwill and Intangibles

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