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Stock Option Plans (Tables) | 3 Months Ended
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Mar. 31, 2017
Text Block [Abstract] |
Black-Scholes Option Pricing Model with Weighted Average | For purposes of expense recognition, the estimated fair values of the stock option grants are amortized to expense over the options’ vesting period. We estimated the fair value of stock options at the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions:
|  | 2017 |  |  | 2016 |
Expected dividend yield |  |  | 2.8 | % |  |  | 3.0 | %
Expected risk-free interest rate |  |  | 2.3 | % |  |  | 1.6 | %
Volatility |  |  | 27.2 | % |  |  | 27.7 | %
Expected life (in years) |  |  | 5.0 |  |  |  | 5.5 |
Stock Option Activity and Related Information | The following is a summary of our stock option activity and related information for 2017 (in millions, except exercise price and year data):
|  | Three-month period ended March 31, 2017 |
| Shares Under Option |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Contractual Term (in years) |  |  | Aggregate Intrinsic Value |
Beginning balance |  |  | 10.3 |  |  | $ | 41.40 |  |  |  |  |  |  |  |  |
Granted |  |  | 1.6 |  |  |  | 56.86 |  |  |  |  |  |  |  |  |
Exercised |  |  | (0.7 | ) |  |  | 30.98 |  |  |  |  |  |  |  |  |
Forfeited or canceled |  |  | — |  |  |  | — |  |  |  |  |  |  |  |  |
Ending balance |  |  | 11.2 |  |  | $ | 44.35 |  |  |  | 4.53 |  |  | $ | 137.4 |
Exercisable at end of period |  |  | 3.2 |  |  | $ | 37.28 |  |  |  | 2.38 |  |  | $ | 61.5 |
Ending vested and expected to vest |  |  | 11.0 |  |  | $ | 44.26 |  |  |  | 4.50 |  |  | $ | 135.6 |
Other Information Regarding Stock Options Outstanding and Exercisable | Other information regarding stock options outstanding and exercisable at March 31, 2017 is summarized as follows (in millions, except exercise price and year data):
|  |  |  |  |  |  |  | Options Outstanding |  |  | Options Exercisable |
Range of Exercise Prices |  |  | Number Outstanding |  |  | Weighted Average Remaining Contractual Term (in years) |  |  | Weighted Average Exercise Price |  |  | Number Exercisable |  |  | Weighted Average Exercise Price |
$ | 23.76 |  |  | - |  | $ | 39.17 |  |  |  | 3.2 |  |  |  | 2.17 |  |  | $ | 35.68 |  |  |  | 2.6 |  |  | $ | 35.01 |
| 43.71 |  |  | - |  |  | 46.17 |  |  |  | 4.5 |  |  |  | 5.53 |  |  |  | 44.77 |  |  |  | — |  |  |  | — |
| 46.87 |  |  | - |  |  | 49.55 |  |  |  | 1.9 |  |  |  | 3.97 |  |  |  | 46.89 |  |  |  | 0.6 |  |  |  | 46.87 |
| 56.86 |  |  | - |  |  | 56.86 |  |  |  | 1.6 |  |  |  | 6.96 |  |  |  | 56.86 |  |  |  | — |  |  |  | — |
$ | 23.76 |  |  | - |  | $ | 56.86 |  |  |  | 11.2 |  |  |  | 4.53 |  |  | $ | 44.35 |  |  |  | 3.2 |  |  | $ | 37.28 |

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