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Stock Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Share-based Compensation, Activity [Table Text Block] | Following is a summary of stock option activity for the year ended December 31, 2018, relating to the stock options awarded under the 2018 LTIP, the 2009 LTIP and the 1993 Plan:
| Shares |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Term (Years) |  | Aggregate Intrinsic Value(a) (In thousands)
Options outstanding at December 31, 2017 | 1,822,818 |  |  | $ | 20.48 |  |  | 6.88 |  | $ | 25,376 |
Granted | 438,500 |  |  | 31.89 |  |  |  |  |
Exercised | (38,628 | ) |  | 13.92 |  |  |  |  |
Forfeited | (10,000 | ) |  | 28.49 |  |  |  |  |
Options outstanding at December 31, 2018 | 2,212,690 |  |  | $ | 22.82 |  |  | 6.58 |  | $ | 3,123 |
Options vested or expected to vest at December 31, 2018 | 2,151,750 |  |  | $ | 22.74 |  |  | 6.53 |  | $ | 3,073 |
Options exercisable at December 31, 2018 | 1,400,490 |  |  | $ | 21.22 |  |  | 5.65 |  | $ | 2,486 |
(a)Intrinsic value is defined as the amount by which the fair value of the underlying common shares exceeds the exercise price of the option.
Five Year Service Stock Options [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The fair value of our five-year service-based stock options granted during the years ended December 31, 2018, 2017 and 2016 was established at the date of grant using the Black-Scholes pricing model, with the weighted average assumptions as follows:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
Risk-free interest rate | 2.72 | % |  | 1.96 | % |  | 1.34 | %
Expected volatility | 32.01 | % |  | 39.49 | % |  | 47.20 | %
Expected term (in years) | 5.7 |  |  | 5.9 |  |  | 5.7 |
Weighted average grant date fair value of options granted during the period | $ | 11.31 |  |  | $ | 9.45 |  |  | $ | 7.57 |

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- Definition Tabular disclosure for share-based compensation 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109197908&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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