Stock Option and Purchase Plans - Summary of Weighted Average Fair Value of Options Granted (Detail) - $ / shares |
12 Months Ended | ||
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Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
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| Disclosure of Compensation Related Costs, Share-based Payments [Abstract] | |||
| Weighted Average Fair Value of the Options Granted (in usd per share) | $ 17.60 | $ 16.85 | $ 18.00 |
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- References No definition available.
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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