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Shareholders' Equity - Schedule of Stock Repurchases (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 78 Months Ended
--- | --- | --- | --- | ---
Oct. 26, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018
Class of Stock [Line Items] |  |  |  |  |  |
Shares purchased (in shares) |  |  |  |  |  | 117.0
Total repurchase cost |  |  |  | $ 1,256.0 | $ 1,913.0 | $ 10,800.0
2012 - Share Repurchase Program |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Shares purchased (in shares) |  | 2.1 | 5.4 | 8.8 | 14.5 |
Average price per share of stock repurchased (in dollars per share) |  | $ 145.71 | $ 139.61 | $ 142.15 | $ 131.58 |
Total repurchase cost |  | $ 300.0 | $ 749.0 | $ 1,250.0 | $ 1,903.0 |
Additional associated costs |  | 1.0 | 4.0 | 6.0 | 10.0 |
Total repurchase and associated costs |  | $ 301.0 | $ 753.0 | $ 1,256.0 | $ 1,913.0 |
Subsequent Event | 2012 - Share Repurchase Program |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Shares purchased (in shares) | 0.2 |  |  |  |  |
Average price per share of stock repurchased (in dollars per share) | $ 146.52 |  |  |  |  |
Total repurchase cost | $ 24.2 |  |  |  |  |

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