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Non-controlling interest - Changes in Non-controlling Interest (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Jul. 01, 2018 | Dec. 31, 2020 | Dec. 31, 2019
Changes in non-controlling interest [Roll Forward] |  |  |  |
Balance, beginning of period |  |  | $ 151 | $ 152
Net (loss)/income attributable to non-controlling interest | $ (2) | $ (6) | (3) | (1)
Share buyback of Heirs Holding shares in Seadrill Nigeria Operations |  | $ 0 | (11) |
Deconsolidation of Ship Finance SPV's |  |  | (137) |
Balance, end of period | 152 |  | 0 | 151
North Atlantic Drilling Ltd |  |  |  |
Changes in non-controlling interest [Roll Forward] |  |  |  |
Balance, beginning of period |  |  | 0 | 0
Net (loss)/income attributable to non-controlling interest |  |  | 0 | 0
Share buyback of Heirs Holding shares in Seadrill Nigeria Operations |  |  | 0 |
Deconsolidation of Ship Finance SPV's |  |  | 0 |
Balance, end of period | 0 |  | 0 | 0
Sevan Drilling Limited |  |  |  |
Changes in non-controlling interest [Roll Forward] |  |  |  |
Balance, beginning of period |  |  | 0 | 0
Net (loss)/income attributable to non-controlling interest |  |  | 0 | 0
Share buyback of Heirs Holding shares in Seadrill Nigeria Operations |  |  | 0 |
Deconsolidation of Ship Finance SPV's |  |  | 0 |
Balance, end of period | 0 |  | 0 | 0
Asia Offshore Drilling Ltd |  |  |  |
Changes in non-controlling interest [Roll Forward] |  |  |  |
Balance, beginning of period |  |  | 0 | 0
Net (loss)/income attributable to non-controlling interest |  |  | 0 | 0
Share buyback of Heirs Holding shares in Seadrill Nigeria Operations |  |  | 0 |
Deconsolidation of Ship Finance SPV's |  |  | 0 |
Balance, end of period | 0 |  | 0 | 0
Ship Finance SPV's |  |  |  |
Changes in non-controlling interest [Roll Forward] |  |  |  |
Balance, beginning of period |  |  | 140 | 145
Net (loss)/income attributable to non-controlling interest |  |  | (3) | (5)
Share buyback of Heirs Holding shares in Seadrill Nigeria Operations |  |  | 0 |
Deconsolidation of Ship Finance SPV's |  |  | (137) |
Balance, end of period | 145 |  | 0 | 140
Seadrill Nigeria Operations Limited |  |  |  |
Changes in non-controlling interest [Roll Forward] |  |  |  |
Balance, beginning of period |  |  | 11 | 7
Net (loss)/income attributable to non-controlling interest |  |  | 0 | 4
Share buyback of Heirs Holding shares in Seadrill Nigeria Operations |  |  | (11) |
Deconsolidation of Ship Finance SPV's |  |  | 0 |
Balance, end of period | $ 7 |  | $ 0 | $ 11

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