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Employee Stock Plans (Tables) | 12 Months Ended
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Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Share-based Compensation, Stock Options, Activity [Table Text Block] | The following information relates to the stock option activity for the year ended December 31, 2018:
| Shares |  | Weighted- Average Exercise Price |  | Weighted- Average Remaining Contractual Life |  | Aggregate Intrinsic Value
Outstanding at December 31, 2017 | 1,540,369 |  |  | $ | 58.34 |  |  |  |  |  |  |
Granted | 332,928 |  |  |  | 81.88 |  |  |  |  |  |  |
Exercised | (176,783 | ) |  |  | 49.86 |  |  |  |  |  |  |
Forfeited | (64,537 | ) |  |  | 67.79 |  |  |  |  |  |  |
Outstanding at December 31, 2018 | 1,631,977 |  |  |  | 63.69 |  |  | 81 | months |  | $ | 14,401 |
Exercisable at December 31, 2018 | 787,871 |  |  | $ | 54.75 |  |  | 65 | months |  | $ | 11,579 |
The aggregate intrinsic value in the table above represents the total pre-tax intrinsic value (the difference between the company's closing stock price on the last trading day of 2018 and the exercise price, multiplied by the number of in-the-money options) received by the option holders had all option holders exercised their options on December 31, 2018. This amount changes based on the market value of the company's stock.
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The fair value of stock options was estimated using the Black-Scholes valuation model with the following weighted-average assumptions for the years ended December 31:
| 2018 |  | 2017 |  | 2016
Volatility (percent) (a) | 24 |  | 26 |  | 31
Expected term (in years) (b) | 5.5 |  | 5.1 |  | 5.2
Risk-free interest rate (percent) (c) | 2.7 |  | 1.9 |  | 1.3
(a) | Volatility is measured using historical daily price changes of the company's common stock over the expected term of the option.
(b) | The expected term represents the weighted-average period the option is expected to be outstanding and is based primarily on the historical exercise behavior of employees.
(c) | The risk-free interest rate is based on the U.S. Treasury zero-coupon yield with a maturity that approximates the expected term of the option.
Schedule of Nonvested Share Activity [Table Text Block] | The following information summarizes the changes in non-vested performance shares, performance units, restricted stock, and restricted stock units for 2018:
| Shares |  | Weighted- Average Grant Date Fair Value
Non-vested shares at December 31, 2017 | 1,176,965 |  |  | $ | 62.60 |
Granted | 538,712 |  |  |  | 77.97 |
Vested | (523,477 | ) |  |  | 62.49 |
Forfeited | (99,335 | ) |  |  | 66.50 |
Non-vested shares at December 31, 2018 | 1,092,865 |  |  | $ | 69.87 |

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 3: 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=96867065&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=96867065&loc=d3e5070-113901 + Details
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