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Stock-Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2014
Summary of Activity Under Stock Option Plans | A summary of the activity under our stock option plans as of March 31, 2014 and changes during the three month period then ended, is presented below:
| Options Outstanding |  | Weighted- Average Exercise Price Per Share |  | Weighted-Average Remaining Contractual Life in Years |  | Aggregate Intrinsic Value
Options outstanding at December 31, 2013 | 893,139 |  |  | $ | 43.23 |  |  | 3.9 |  | $ | 16,403,816 |
Options granted | — |  |  | — |  |  |  |  |  |
Options exercised | (235,883 | ) |  | 45.45 |  |  |  |  |  |
Options forfeited | (750 | ) |  | 40.70 |  |  |  |  |  |
Options outstanding at March 31, 2014 | 656,506 |  |  | 42.44 |  |  | 4.1 |  | 13,184,080 |
Options exercisable at March 31, 2014 | 568,073 |  |  | 41.97 |  |  | 3.6 |  | 11,670,746 |
Options vested or expected to vest at March 31, 2014* | 654,581 |  |  | 42.43 |  |  | 4.1 |  | 13,138,680 |
* In addition to the vested options, we expect a portion of the unvested options to vest at some point in the future. Options expected to vest are calculated by applying an estimated forfeiture rate to the unvested options.
Assumptions Used in Calculation of Fair Value | Below are the assumptions used in the Monte Carlo calculation:
| March 31, 2014 |  | March 31, 2013
Expected volatility | 33.7% |  | 37.1%
Expected term (in years) | 3.0 |  | 3.0
Risk-free interest rate | 0.67% |  | 0.40%
Expected dividend yield | — |  | —
Performance-Based Restricted Stock [Member] |
Restricted Stock Activities |
| Performance-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2013 | 71,175 |
Awards granted | 47,200 |
Stock issued | (12,503 | )
Awards forfeited | 2,760 |
Non-vested awards outstanding at March 31, 2014 | 108,632 |
Time Based Restricted Stock [Member] |
Restricted Stock Activities |
| Time-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2013 | 231,026 |
Awards granted | 62,050 |
Stock issued | (20,952 | )
Awards forfeited | (1,755 | )
Non-vested awards outstanding at March 31, 2014 | 270,369 |
Deferred Stock Units [Member] |
Restricted Stock Activities |
| Deferred Stock Units
Awards outstanding at December 31, 2013 | 31,550 |
Awards granted | — |
Stock issued | — |
Awards outstanding at March 31, 2014 | 31,550 |

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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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- Definition Tabular disclosure of the number and weighted-average exercise prices (or conversion ratios) for share options (or share units) that were outstanding at the beginning and end of the year, vested and expected to vest, exercisable or convertible at the end of the year, and the number of share options or share units that were granted, exercised or converted, forfeited, and expired during the year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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