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Stockholders' Equity (Details) - $ / shares | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 31, 2017 | May 31, 2015
Stockholders' equity |  |  |  |  |
Common stock, shares authorized (in shares) | 153,000,000 | 153,000,000 |  |  |
Common Stock |  |  |  |  |
Share repurchase activity |  |  |  |  |
Shares repurchased (in shares) | 2,400,000 | 200,000 | 1,700,000 |  |
Average price (in usd per share) | $ 173.19 | $ 161.16 | $ 209.39 |  |
Common Stock | January 2017 Plan |  |  |  |  |
Stockholders' equity |  |  |  |  |
Stock repurchase program, number of shares authorized (in shares) |  |  |  | 1,900,000 |
Share repurchase activity |  |  |  |  |
Stock repurchase plan, number of remaining shares to be repurchased (in shares) | 1,600,000 |  |  |  |
Common Stock | May 2015 Plan |  |  |  |  |
Stockholders' equity |  |  |  |  |
Stock repurchase program, number of shares authorized (in shares) |  |  |  |  | 3,000,000
Share repurchase activity |  |  |  |  |
Stock repurchase plan, number of remaining shares to be repurchased (in shares) | 0 |  |  |  |
Voting Common Stock |  |  |  |  |
Stockholders' equity |  |  |  |  |
Common stock, shares authorized (in shares) | 150,000,000.0 |  |  |  |
Class B Non-Voting Common Stock |  |  |  |  |
Stockholders' equity |  |  |  |  |
Common stock, shares authorized (in shares) | 3,000,000.0 |  |  |  |
Preferred Stock |  |  |  |  |
Share repurchase activity |  |  |  |  |
Preferred stock, shares authorized (in shares) | 5,000,000.0 |  |  |  |

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