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Income Taxes - Gross Unrecognized Tax Benefits (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Mar. 31, 2018 | Mar. 31, 2017 | Mar. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |  |
Unrecognized tax benefits at beginning of period |  | $ 486 | $ 555 | $ 616
Additions based on tax positions related to prior years |  | 47 | 7 | 116
Reductions based on tax positions related to prior years |  | (124) | (67) | (62)
Additions based on tax positions related to current year |  | 778 | 105 | 28
Reductions based on settlements | $ (39) | (7) | (113) | (141)
Reductions based on the lapse of the applicable statutes of limitations |  | 0 | 0 | (6)
Exchange rate fluctuations |  | 3 | (1) | 4
Unrecognized tax benefits at end of period |  | $ 1,183 | $ 486 | $ 555

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- Definition Unrecognized Tax Benefits, Increase (Decrease) Resulting from Foreign Currency Translation + References No definition available. + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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