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Investment in Change Healthcare Joint Venture (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 01, 2019 | Dec. 31, 2019 | Sep. 30, 2019 | Dec. 31, 2019 | Dec. 31, 2020 | Sep. 30, 2020 | Mar. 31, 2020 | Jun. 30, 2019 | Apr. 01, 2019 | Mar. 31, 2019
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Shareholders' equity |  | $ 6,385,000,000 | $ 6,692,000,000 | $ 6,385,000,000 | $ (277,000,000) | $ 6,090,000,000 | $ 5,309,000,000 |  |  | $ 8,287,000,000
Change Healthcare, Inc. |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance Initial Public Offering | $ 888,000,000 |  |  |  |  |  |  |  |  |
Proceeds from concurrent offering of other securities | $ 279,000,000 |  |  |  |  |  |  |  |  |
Change Healthcare, Inc. | IPO |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Offering price (in usd per share) | $ 13 |  |  |  |  |  |  |  |  |
Transition Services Agreements (“TSA”) |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Fees earned from TSA |  |  |  | 18,000,000 |  |  |  |  |  |
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% |  |  |  |  |
Receivable recognized |  |  |  |  | $ 0 |  |  |  |  |
Joint Venture |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) |  |  |  |  |  |  |  | 70.00% |  |
Change Healthcare JV | Change Healthcare, Inc. |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Contributions made in exchange for additional membership interests | $ 609,000,000 |  |  |  |  |  |  |  |  |
Ownership interest (percent) | 41.50% |  |  |  |  |  |  |  |  |
Change Healthcare JV | Joint Venture |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) | 58.50% |  |  |  |  |  |  |  |  |
Pre-tax dilution loss |  |  | 246,000,000 | 246,000,000 |  |  |  |  |  |
Impairment |  |  | 1,200,000,000 | 1,200,000,000 |  |  |  |  |  |
Proportionate loss from investment in Change Healthcare |  | 28,000,000 |  | 75,000,000 |  |  |  |  |  |
Retained Earnings |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Shareholders' equity |  | $ 12,075,000,000 | $ 11,965,000,000 | $ 12,075,000,000 | $ 7,595,000,000 | $ 13,890,000,000 | 13,022,000,000 |  |  | 12,409,000,000
Opening retained earnings adjustment: adoption of new accounting standard |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Shareholders' equity |  |  |  |  |  |  | (13,000,000) |  |  | 11,000,000
Opening retained earnings adjustment: adoption of new accounting standard | Retained Earnings |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Shareholders' equity |  |  |  |  |  |  | $ (13,000,000) |  |  | $ 11,000,000
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] |  |  |  |  |  |  |  |  |  |
Shareholders' equity |  |  |  |  |  |  |  |  | $ 80,000,000 |

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