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Investment in Change Healthcare Joint Venture (Details) - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 01, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Jun. 30, 2020 | Mar. 31, 2020 | Jun. 30, 2019 | Apr. 01, 2019 | Mar. 31, 2019
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative-effect adjustment to beginning retained earnings |  | $ 6,090 | $ 6,692 | $ 6,090 | $ 6,692 | $ 5,653 | $ 5,309 | $ 8,068 |  | $ 8,287
Proportionate loss from investment in Change Healthcare |  | $ 0 | 1,454 | $ 0 | 1,450 |  |  |  |  |
Change Healthcare, Inc. |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance Initial Public Offering | $ 888 |  |  |  |  |  |  |  |  |
Proceeds from concurrent offering of other securities | $ 279 |  |  |  |  |  |  |  |  |
Change Healthcare, Inc. | IPO |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Offering price (in usd per share) | $ 13 |  |  |  |  |  |  |  |  |
Tax Receivable Agreement (“TRA”) |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Percentage of net cash tax savings realized or deemed realized under TRA (percent) |  | 85.00% |  | 85.00% |  |  |  |  |  |
Joint Venture |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) |  |  |  |  |  |  |  | 70.00% |  |
Pre-tax dilution loss |  |  | 246 |  | 246 |  |  |  |  |
Change Healthcare JV | Change Healthcare, Inc. |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Contributions made in exchange for additional membership interests | $ 609 |  |  |  |  |  |  |  |  |
Ownership interest (percent) | 41.50% |  |  |  |  |  |  |  |  |
Change Healthcare JV | Joint Venture |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) | 58.50% |  |  |  |  |  |  |  |  |
Impairment |  |  | 1,200 |  | 1,200 |  |  |  |  |
Proportionate loss from investment in Change Healthcare |  |  | 51 |  | 47 |  |  |  |  |
Retained Earnings |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative-effect adjustment to beginning retained earnings |  | $ 13,890 | $ 11,965 | $ 13,890 | $ 11,965 | $ 13,384 | 13,022 | $ 12,770 |  | 12,409
Opening retained earnings adjustment: adoption of new accounting standard |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative-effect adjustment to beginning retained earnings |  |  |  |  |  |  | (13) |  |  | 11
Opening retained earnings adjustment: adoption of new accounting standard | Retained Earnings |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative-effect adjustment to beginning retained earnings |  |  |  |  |  |  | $ (13) |  |  | $ 11
Opening retained earnings adjustment: adoption of new accounting standard | ASU 2014-09 | Retained Earnings | Change Healthcare JV | Joint Venture |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative-effect adjustment to beginning retained earnings |  |  |  |  |  |  |  |  | $ 80 |

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