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Stock Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
Stock-Based Compensation [Abstract] |
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block] | The following table summarizes the Company's pre-tax compensation expense and related income tax benefit for the years ended December 31, 2017, 2016 and 2015 related to the Company's performance units and restricted stock.
Year Ended December 31 (In millions) | 2017 | 2016 | 2015
Performance units: |  |  |
Total shareholder return | $ | 7.6 |  | $ | 4.5 |  | $ | 7.6 |
Earnings per share | 1.4 |  | — |  | 0.7 |
Total performance units | 9.0 |  | 4.5 |  | 8.3 |
Restricted stock | 0.1 |  | 0.1 |  | 0.1 |
Total compensation expense | 9.1 |  | 4.6 |  | 8.4 |
Less: Amount paid by unconsolidated affiliates | — |  | — |  | 0.5 |
Net compensation expense | $ | 9.1 |  | $ | 4.6 |  | $ | 7.9 |
Income tax benefit | $ | 3.5 |  | $ | 1.8 |  | $ | 3.1 |
Performance Units Total Shareholder Return Valuation Assumptions [Table Text Block] | The number of performance units granted based on total shareholder return and the assumptions used to calculate the grant date fair value of the performance units based on total shareholder return are shown in the following table.
| 2017 | 2016 | 2015
Number of units granted | 260,570 |  | 284,211 |  | 264,454 |
Fair value of units granted | $ | 41.77 |  | $ | 20.97 |  | $ | 31.02 |
Expected dividend yield | 3.8 | % | 3.5 | % | 2.6 | %
Expected price volatility | 19.9 | % | 19.8 | % | 16.9 | %
Risk-free interest rate | 1.44 | % | 0.88 | % | 0.91 | %
Expected life of units (in years) | 2.80 |  | 2.84 |  | 2.85 |
Performance Units Earnings Per Share Valuation Assumptions [Table Text Block] | The number of performance units granted based on earnings per share and the grant date fair value are shown in the following table.
| 2017 | 2016 | 2015
Number of units granted | 86,857 |  | 94,735 |  | 88,156 |
Fair value of units granted | $ | 34.83 |  | $ | 26.64 |  | $ | 33.99 |
Restricted Stock Valuation Assumptions [Table Text Block] | The number of shares of restricted stock granted and the grant date fair value are shown in the following table.
| 2017 | 2016 | 2015
Shares of restricted stock granted | 3,145 |  | 1,881 |  | 958 |
Fair value of restricted stock granted | $ | 34.96 |  | $ | 29.27 |  | $ | 26.11 |
Share-based Compensation, Activity [Table Text Block] | A summary of the activity for the Company's performance units and restricted stock at December 31, 2017 and changes in 2017 are shown in the following table.
| Performance Units |  |
| Total Shareholder Return | Earnings Per Share | Restricted Stock
(Dollars in millions) | Number of Units |  | Aggregate Intrinsic Value | Number of Units |  | Aggregate Intrinsic Value | Number of Shares | Aggregate Intrinsic Value
Units/shares outstanding at 12/31/16 | 664,045 |  |  |  | 221,350 |  |  |  | 4,912 |  |
Granted | 260,570 |  | (A) |  | 86,857 |  | (A) |  | 3,145 |  |
Converted | (185,214 | ) | (B) | $ | — |  | (61,742 | ) | (B) | $ | — |  | N/A |  |
Vested | N/A |  |  |  | N/A |  |  |  | (3,815 | ) | $ | 0.1 |
Forfeited | (14,850 | ) |  |  | (4,947 | ) |  |  | — |  |
Units/shares outstanding at 12/31/17 | 724,551 |  |  | $ | 2.4 |  | 241,518 |  |  | $ | 7.2 |  | 4,242 |  | $ | 0.1 |
Units/shares fully vested at 12/31/17 | 201,431 |  |  | $ | — |  | 67,148 |  |  | $ | 1.2 |  |  |
(A) | For performance units, this represents the target number of performance units granted. Actual number of performance units earned, if any, is dependent upon performance and may range from zero percent to 200 percent of the target.
(B) | These amounts represent performance units that vested at December 31, 2016 which were settled in February 2017.
Schedule of Nonvested Share Activity [Table Text Block] | A summary of the activity for the Company's non-vested performance units and restricted stock at December 31, 2017 and changes in 2017 are shown in the following table.
| Performance Units |  |
| Total Shareholder Return | Earnings Per Share | Restricted Stock
| Number of Units |  | Weighted-Average Grant Date Fair Value | Number of Units |  | Weighted-Average Grant Date Fair Value | Number of Shares | Weighted-Average Grant Date Fair Value
Units/shares non-vested at 12/31/16 | 478,831 |  |  | $ | 25.16 |  | 159,608 |  |  | $ | 29.71 |  | 4,912 |  | $ | 31.29 |
Granted | 260,570 |  | (A) | $ | 41.77 |  | 86,857 |  | (A) | $ | 34.83 |  | 3,145 |  | $ | 34.96 |
Vested | (201,431 | ) |  | $ | 31.18 |  | (67,148 | ) |  | $ | 33.99 |  | (3,815 | ) | $ | 31.71 |
Forfeited | (14,850 | ) |  | $ | 31.01 |  | (4,947 | ) |  | $ | 31.12 |  | — |  | $ | — |
Units/shares non-vested at 12/31/17 | 523,120 |  |  | $ | 30.96 |  | 174,370 |  |  | $ | 30.58 |  | 4,242 |  | $ | 33.58 |
Units/shares expected to vest | 492,446 |  | (B) |  | 164,148 |  | (B) |  | 4,242 |  |
(A) | For performance units, this represents the target number of performance units granted. Actual number of performance units earned, if any, is dependent upon performance and may range from zero percent to 200 percent of the target.
(B) | The intrinsic value of the performance units based on total shareholder return and earnings per share is $2.3 million and $5.7 million, respectively.
Fair Value of Vested Performance Units and Restricted Stock [Table Text Block] | A summary of the Company's fair value for its vested performance units and restricted stock is shown in the following table.
Year Ended December 31 (In millions) | 2017 | 2016 | 2015
Performance units: |  |  |
Total shareholder return | $ | 6.3 |  | $ | 6.4 |  | $ | 8.5 |
Earnings per share | 1.2 |  | — |  | — |
Restricted stock | 0.1 |  | 0.1 |  | 0.2 |
Schedule of Unrecognized Compensation Cost, Nonvested Awards [Table Text Block] | A summary of the Company's unrecognized compensation cost for its non-vested performance units and restricted stock and the weighted-average periods over which the compensation cost is expected to be recognized are shown in the following table.
December 31, 2017 | Unrecognized Compensation Cost (In millions) | Weighted Average to be Recognized (In years)
Performance units: |  |
Total shareholder return | $ | 8.5 |  | 1.75
Earnings per share | 2.6 |  | 1.67
Total performance units | 11.1 |  |
Restricted stock | 0.1 |  | 1.34
Total | $ | 11.2 |  |

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- Definition Tabular disclosure of components of a stock option or other award plan under which equity-based compensation is awarded to employees, typically comprised of the amount of unearned compensation (deferred compensation cost), compensation expense, and changes in the quantity and fair value of the shares (or other type of equity) granted, exercised, forfeited, and issued and outstanding pertaining to that plan. Disclosure may also include nature and general terms of such arrangements that existed during the period and potential effects of those arrangements on shareholders, effect of compensation cost arising from equity-based payment arrangements on the income statement, method of estimating the fair value of the goods or services received, or the fair value of the equity instruments granted, during the period, cash flow effects resulting from equity-based payment arrangements and, for registrants that accelerate vesting of out of the money share options, reasons for the decision to accelerate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -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 1 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure of the changes in outstanding nonvested shares. + 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)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -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 (d) -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 (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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