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Stock-based Compensation Expense Restricted and Performance Stock Unit activity (Tables) | 12 Months Ended
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Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures [Abstract] |
Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] | RSU activity is summarized in the following table:
| Units |  | Weighted Average Grant Date Fair Value
Outstanding as of December 31, 2014 | 463,893 |  |  | $ | 28.96 |
Granted | 181,797 |  |  | 34.77 |
Forfeited | (14,988 | ) |  | 34.10 |
Vested | (187,709 | ) |  | 25.82 |
Outstanding as of December 31, 2015 | 442,993 |  |  | 32.84 |
Granted | 193,734 |  |  | 35.89 |
Forfeited | (3,044 | ) |  | 28.62 |
Vested | (174,891 | ) |  | 31.47 |
Outstanding as of December 31, 2016 | 458,792 |  |  | 34.68 |
Granted | 202,145 |  |  | 41.96 |
Forfeited | (64,840 | ) |  | 39.57 |
Vested | (196,721 | ) |  | 31.78 |
Outstanding as of December 31, 2017 | 399,376 |  |  | 37.98 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The fair value of stock-based compensation related to the TSR component of performance-based RSUs was determined using the Monte Carlo model and the following weighted average assumptions:
| 2017 |  | 2016
Risk-free interest rate |  |  | 1.5 | % |  |  |  | 0.9 | %
Expected dividend yield |  |  | — | % |  |  |  | — | %
Expected term (in years) |  |  | 3.0 |  |  |  |  | 3.0 |
Volatility | 15.6 | % | - | 22.9 | % |  | 14.5 | % | - | 25.9 | %

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- Definition Tabular disclosure of the number and weighted-average grant date fair value for restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock units that were granted, vested, or forfeited 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=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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