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Stock-based Compensation Expense Restricted and Performance Stock Unit activity (Tables) | 12 Months Ended
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Dec. 31, 2013
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] | Restricted stock unit activity is summarized in the following table:
| Units |  | Weighted Average Grant Date Fair Value
Outstanding as of December 31, 2010 | 465,428 |  |  | $ | 17.88 |
Granted | 152,657 |  |  | 23.84 |
Forfeited | (106,979 | ) |  | 22.35 |
Vested | (19,702 | ) |  | 23.34 |
Outstanding as of December 31, 2011 | 491,404 |  |  | 18.54 |
Granted | 186,495 |  |  | 24.72 |
Forfeited | (22,947 | ) |  | 18.95 |
Vested | (214,390 | ) |  | 15.67 |
Outstanding as of December 31, 2012 | 440,562 |  |  | 22.54 |
Granted | 183,071 |  |  | 29.25 |
Forfeited | (7,007 | ) |  | 27.15 |
Vested | (185,536 | ) |  | 20.20 |
Outstanding as of December 31, 2013 | 431,090 |  |  | 26.31 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The assumptions used in the Monte Carlo model are summarized as follows:
| 2013
Stock price at March 5, 2013 |  |  | $ | 30.29 |
Risk-free rate |  |  | 0.34 | %
Expected term (in years) |  |  | 3 |
Expected volatility |  |  | 16.77 | %
Range of expected volatility for EEI Index | 12.06 | % | - | 25.13 | %
Dividend yield |  |  | 0 | %

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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=6415400&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=6415400&loc=d3e5070-113901 + Details
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