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Common Stock and Treasury Stock - Additional Information (Detail) - USD ($) | 6 Months Ended | 70 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2018 | Feb. 28, 2018
Stock authorized to purchase under stock repurchase program |  |  | $ 200,000,000
Repurchase of common stock, shares | 2,346,427 | 44,129,393 |
Repurchase of common stock, value | $ 54,500,000 | $ 547,800,000 |
Maximum [Member] |  |  |
Remaining value of shares authorized for purchase under the stock repurchase program | $ 176,600,000 | $ 176,600,000 |

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