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Stock Option Plan - Summary of Option Activity (Detail) (USD $)
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12 Months Ended | ||
|---|---|---|---|
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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| Shares | |||
| Outstanding at January 1 (in shares) | 570,840 | 674,585 | 532,881 |
| Granted (in shares) | 237,500 | 277,500 | 195,000 |
| Exercised (in shares) | (49,715) | (149,995) | (40,796) |
| Expired/Forfeited (in shares) | (5,000) | (231,250) | (12,500) |
| Outstanding December 31 (in shares) | 753,625 | 570,840 | 674,585 |
| Exercisable at December 31 (in shares) | 413,000 | 377,715 | 512,085 |
| Weighted Average Exercise Price | |||
| Outstanding at January 1 (in usd per share) | $ 41.04 | $ 40.40 | $ 39.12 |
| Granted (in usd per share) | $ 44.42 | $ 39.29 | $ 41.82 |
| Exercised (in usd per share) | $ 33.15 | $ 31.03 | $ 29.03 |
| Expired/Forfeited (in usd per share) | $ 52.16 | $ 43.56 | $ 45.05 |
| Outstanding December 31 (in usd per share) | $ 42.55 | $ 41.04 | $ 40.40 |
| Exercisable at December 31 (in usd per share) | $ 42.42 | $ 41.41 | $ 39.96 |