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Treasury Stock - Additional Information (Detail) - USD ($) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Mar. 31, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Treasury stock, shares | 6,405,000 |  |  | 6,405,000 |  | 6,405,000
Treasury stock, cost | $ 475,184,000 |  |  | $ 475,184,000 |  | $ 475,184,000
Class A | 2005 Share Repurchase Program |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Shares repurchased program expiration date |  | Apr. 29, 2018 |  |  |  |
Stock authorized to repurchase by board of directors | $ 800,000,000 |  |  | $ 800,000,000 |  |
Treasury stock, shares | 3,826,947 |  |  | 3,826,947 |  |
Treasury stock, shares repurchased | 0 |  | 0 | 0 | 0 |
Shares repurchased, average price per share |  |  |  | $ 90.23 |  |
Treasury stock, cost | $ 345,300,000 |  |  | $ 345,300,000 |  |
Class A | 2005 Share Repurchase Program | Board of Directors |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Shares repurchased program expiration date |  |  |  | Apr. 29, 2023 |  |

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