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Shareholders' Equity - Schedule of Stock Repurchases (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions | 3 Months Ended | 4 Months Ended | 9 Months Ended | 90 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Oct. 25, 2019 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |
Total repurchase cost | $ 350.0 | $ 1,056.0 | $ 101.0 | $ 301.0 | $ 402.0 | $ 553.0 |  |  |  |
2012 - Share Repurchase Program |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |
Shares purchased (in shares) | 1,800 |  |  | 2,100 |  |  |  | 8,200 | 8,800 | 126,500
Average price per share of stock repurchased (in dollars per share) | $ 190.83 |  |  | $ 145.71 |  |  |  | $ 183.26 | $ 142.15 |
Total repurchase cost | $ 350.0 |  |  | $ 300.0 |  |  |  | $ 1,500.0 | $ 1,250.0 | $ 12,500.0
Additional associated costs | 0.0 |  |  | 1.0 |  |  |  | 7.0 | 6.0 |
Total costs recorded to retained earnings | $ 350.0 |  |  | $ 301.0 |  |  |  | $ 1,507.0 | $ 1,256.0 |
Subsequent Event | 2012 - Share Repurchase Program |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |
Shares purchased (in shares) |  |  |  |  |  |  | 25 |  |  |
Average price per share of stock repurchased (in dollars per share) |  |  |  |  |  |  | $ 193.91 |  |  |
Total repurchase cost |  |  |  |  |  |  | $ 4.8 |  |  |

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