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Common Stock (Tables) | 12 Months Ended
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Dec. 31, 2017
Equity [Abstract] |
Summary of Stock Options | The following summarizes options activity:
|  | Years Ended December 31, |
|  | 2017 |  |  | 2016 |  |  | 2015 |
|  | Number |  |  | Average |  |  | Number |  |  | Average |  |  | Number |  |  | Average |
|  | of |  |  | Exercise |  |  | of |  |  | Exercise |  |  | of |  |  | Exercise |
|  | Shares |  |  | Price |  |  | Shares |  |  | Price |  |  | Shares |  |  | Price |
Shares under option at beginning of year |  |  | 17,439,060 |  |  | $ | 54.08 |  |  |  | 15,430,307 |  |  | $ | 59.50 |  |  |  | 10,881,133 |  |  | $ | 61.22 |
Granted |  |  | 6,961,041 |  |  |  | 36.51 |  |  |  | 3,672,411 |  |  |  | 28.26 |  |  |  | 5,746,153 |  |  |  | 54.74 |
Forfeited |  |  | (1,482,531 | ) |  |  | 55.22 |  |  |  | (1,517,065 | ) |  |  | 49.95 |  |  |  | (886,356 | ) |  |  | 62.73 |
Exercised |  |  | (445,523 | ) |  |  | 29.83 |  |  |  | (146,593 | ) |  |  | 28.53 |  |  |  | (310,623 | ) |  |  | 22.56 |
Shares under option at end of year |  |  | 22,472,047 |  |  | $ | 48.99 |  |  |  | 17,439,060 |  |  | $ | 54.08 |  |  |  | 15,430,307 |  |  | $ | 59.50 |
Exercisable at end of year |  |  | 14,309,944 |  |  | $ | 55.00 |  |  |  | 9,828,897 |  |  | $ | 61.56 |  |  |  | 7,498,414 |  |  | $ | 60.30 |
Summary of Stock Option Outstanding Information | The following summarizes information about stock options outstanding at December 31, 2017:
|  | Weighted-Avg |  |  | Options Outstanding |  |  | Options Exercisable |
|  | Remaining |  |  |  |  |  | Weighted-Avg |  |  |  |  |  | Weighted-Avg |
Range of Exercise Price |  | Contractual Life |  |  | Shares |  |  | Exercise Price |  |  | Shares |  |  | Exercise Price |
$23.94 - $55.00 |  |  | 7.55 |  |  |  | 15,797,683 |  |  | $ | 40.25 |  |  |  | 7,635,580 |  |  | $ | 42.19 |
$55.01 - $70.00 |  |  | 5.09 |  |  |  | 4,301,953 |  |  |  | 66.20 |  |  |  | 4,301,953 |  |  |  | 66.20 |
$70.01 - $77.99 |  |  | 3.59 |  |  |  | 2,372,411 |  |  |  | 75.94 |  |  |  | 2,372,411 |  |  |  | 75.94 |
Total |  |  | 6.66 |  |  |  | 22,472,047 |  |  | $ | 48.99 |  |  |  | 14,309,944 |  |  | $ | 55.00 |
Assumption Used in Determination of Fair Value of Share Based Payment Awards | The use of the Black Scholes model requires the use of actual employee exercise activity data and the use of a number of complex assumptions including expected volatility, risk-free interest rate, expected dividends and expected term.
|  | Years Ended December 31, |
|  | 2017 |  |  | 2016 |  |  | 2015 |
Valuation Assumptions: |  |  |  |  |  |  |  |  |  |  |  |
Expected volatility |  |  | 36.1 | % |  |  | 48.6 | % |  |  | 49.1 | %
Risk-free interest rate |  |  | 2.2 | % |  |  | 1.2 | % |  |  | 1.5 | %
Expected dividend yield |  |  | 0.6 | % |  |  | 6.5 | % |  |  | 3.4 | %
Expected term (in years) |  |  | 3.0 |  |  |  | 3.0 |  |  |  | 3.0 |
Summary of Information and Changes in Stock Options with Regard to Stock Option Plans | The following summary presents information regarding outstanding options at December 31, 2017 and changes during 2017 with regard to options under all stock option plans:
|  |  |  |  |  |  |  | Weighted |  |  |  |
|  |  |  |  | Weighted- Average |  |  | Remaining Contractual |  |  |  |
|  | Shares |  |  | Exercise Price |  |  | Term (years) |  |  | Aggregate Intrinsic Value |
Outstanding at December 31, 2016 |  |  | 17,439,060 |  |  | $ | 54.08 |  |  |  | 5.42 |  |  | $ | 6,700,856 |
Granted |  |  | 6,961,041 |  |  | $ | 36.51 |  |  |  |  |  |  |  |  |
Forfeited |  |  | (1,482,531 | ) |  | $ | 55.22 |  |  |  |  |  |  |  |  |
Exercised |  |  | (445,523 | ) |  | $ | 29.83 |  |  |  |  |  |  |  |  |
Outstanding at December 31, 2017 |  |  | 22,472,047 |  |  | $ | 48.99 |  |  |  | 6.66 |  |  | $ | 34,186,368 |
Exercisable at December 31, 2017 |  |  | 14,309,944 |  |  | $ | 55.00 |  |  |  | 5.70 |  |  | $ | 15,557,324 |
Summary of Information Regarding Outstanding SARs | The following summary presents information regarding outstanding SARs:
|  | Year Ended December 31, |
|  | 2017 |  |  | 2016 |
|  | Number |  |  | Average |  |  | Number |  |  | Average |
|  | of |  |  | Exercise |  |  | of |  |  | Exercise |
|  | Shares |  |  | Price |  |  | Shares |  |  | Price |
Shares under SARs at beginning of year |  |  | 4,341,740 |  |  | $ | 28.32 |  |  |  | — |  |  | $ | — |
Granted |  |  | 14,400 |  |  |  | 38.86 |  |  |  | 4,618,400 |  |  |  | 28.32 |
Forfeited |  |  | (283,822 | ) |  |  | 28.35 |  |  |  | (276,660 | ) |  |  | 28.24 |
Exercised |  |  | (2,578,629 | ) |  |  | 34.72 |  |  |  | — |  |  |  | — |
Shares under SARs at end of year |  |  | 1,493,689 |  |  | $ | 28.41 |  |  |  | 4,341,740 |  |  | $ | 28.32 |
Exercisable at end of year |  |  | 75,102 |  |  | $ | 28.33 |  |  |  | — |  |  | $ | — |
Summary of Information Regarding Outstanding Restricted Shares | The following summary presents information regarding outstanding restricted shares:
|  | Years Ended December 31, |
|  | 2017 |  |  | 2016 |  |  | 2015 |
|  |  |  |  | Weighted- |  |  |  |  |  | Weighted- |  |  |  |  |  | Weighted- |
|  | Number |  |  | Average |  |  | Number |  |  | Average |  |  | Number |  |  | Average |
|  | of |  |  | Grant Date |  |  | of |  |  | Grant Date |  |  | of |  |  | Grant Date |
|  | Units |  |  | Fair Value |  |  | Units |  |  | Fair Value |  |  | Units |  |  | Fair Value |
Nonvested at beginning of year |  |  | 4,563,983 |  |  | $ | 41.10 |  |  |  | 1,969,250 |  |  | $ | 61.53 |  |  |  | 1,569,141 |  |  | $ | 73.73 |
Granted |  |  | 1,738,589 |  |  |  | 38.74 |  |  |  | 3,384,325 |  |  |  | 31.59 |  |  |  | 954,075 |  |  |  | 53.27 |
Vested |  |  | (1,018,206 | ) |  |  | 34.84 |  |  |  | (565,202 | ) |  |  | 29.32 |  |  |  | (405,327 | ) |  |  | 54.30 |
Forfeited |  |  | (394,688 | ) |  |  | 55.22 |  |  |  | (224,390 | ) |  |  | 49.95 |  |  |  | (148,639 | ) |  |  | 62.73 |
Nonvested at end of year |  |  | 4,889,678 |  |  | $ | 37.04 |  |  |  | 4,563,983 |  |  | $ | 41.10 |  |  |  | 1,969,250 |  |  | $ | 61.53 |

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