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Summary of Significant Accounting Policies - Redeemable Non Controlling Interest (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Noncontrolling Interest [Line Items] |  |  |
Balance at beginning of period | $ 46 | $ 29 | $ 19
Cash contributions from redeemable noncontrolling interest | 99 | 33 | 27
Distributions to redeemable noncontrolling interest | (65) | (158) | (159)
Comprehensive loss attributable to redeemable noncontrolling interest | (72) | (38) | (17)
Balance at end of period | 78 | 46 | $ 29
Redeemable noncontrolling interest |  |  |
Noncontrolling Interest [Line Items] |  |  |
Distributions to redeemable noncontrolling interest | (2) | (1) |
Non-cash adjustments to redeemable noncontrolling interest | $ 7 | $ 23 |

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- Definition Noncontrolling Interest, Noncash Adjustments + References No definition available. + Details
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- Definition Redeemable Noncontrolling Interest, Cash Contributions from Noncontrolling Interest Holders + References No definition available. + Details
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- Definition Decrease in noncontrolling interest balance from payment of dividends or other distributions by the non-wholly owned subsidiary or partially owned entity, included in the consolidation of the parent entity, to the noncontrolling interest holders. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition Amount of Net Income (Loss) attributable to redeemable noncontrolling interest. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.19) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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- Definition Amount of noncontrolling interests which are redeemable by the parent entity, classified as other equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph 15 -URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph (16)(c) -URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph (12)(c) -URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 480 -SubTopic 10 -Section S99 -Paragraph 3A -Subparagraph 14 -URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764 + Details
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