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Income Taxes - Additional Information (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Mar. 31, 2020
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UNITED KINGDOM |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred Tax Assets, Operating Loss Carryforwards |  | $ 4
Foreign Tax Authority [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred Tax Assets, Operating Loss Carryforwards |  | 6
Deferred Tax Assets, Valuation Allowance |  | 10
Other Assets [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred Tax Assets, Tax Credit Carryforwards, Other |  | 410
Deferred Tax Asset [Domain] | UNITED STATES |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred Tax Assets, Operating Loss Carryforwards |  | 3
Deferred Tax Asset [Domain] | UNITED KINGDOM |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred Tax Assets, Operating Loss Carryforwards |  | 2
Scenario, Plan [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred Tax Assets, Tax Credit Carryforwards, Other |  | $ 205
Forecast [Member] |  |
Operating Loss Carryforwards [Line Items] |  |
Deferred Tax Assets, Tax Credit Carryforwards, Other | $ 205 |

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible alternative minimum tax credit carryforwards. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319 + Details
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