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Stockholders' Equity - Narrative (Details) - USD ($) $ in Millions |  |  |  |  | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Nov. 30, 2015 | Aug. 18, 2015 | May 22, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Aug. 03, 2017
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |
Common stock repurchased and retired during the period (in shares) |  | 1,000,000 | 2,300,000 | 2,000,000 |  |  |  |
Common stock repurchased and retired during the period |  |  |  |  | $ 534.0 | $ 367.5 | $ 241.3 |
Treasury stock acquired (in shares) | 109,207 |  |  |  |  |  |  |
The Share Repurchase Program |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |
Common stock repurchased and retired during the period (in shares) |  |  |  |  | 8,893,993 |  |  |
Common stock repurchased and retired during the period |  |  |  |  | $ 534.0 |  |  |
Additional amount authorized under repurchase program |  |  |  |  |  |  |  | $ 750.0
Amount remaining under repurchase program | $ 858.0 |  |  |  | $ 858.0 |  |  |

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