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Equity - Treasury Stock (Detail) - USD ($) shares in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Jun. 30, 2017
2018 Repurchase Program |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Aggregate authorized amount of common stock to be repurchased | $ 500,000,000.0 |  | $ 500,000,000.0 |  |  |
2017 Repurchase Program |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Treasury stock acquired (in shares) | 0.7 |  | 6.3 |  | 1.4 |
Treasury stock acquired | $ 23,800,000 |  | $ 217,700,000 |  | $ 50,000,000.0 |
2017 Repurchase Program | Maximum |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Aggregate authorized amount of common stock to be repurchased |  |  |  |  |  | $ 300,000,000.0
Treasury Stock Associated with Deferred Compensation Plans |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Treasury stock acquired | $ 0 | $ 0 | $ 3,300,000 | $ 3,400,000 |  |
Common Stock Withheld for Settlement of Employee Tax Liabilities |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |
Treasury stock acquired (in shares) |  |  | 0.4 | 0.5 |  |
Treasury stock acquired |  |  | $ 14,700,000 | $ 18,200,000 |  |

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