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Benefit Plans (Allocation of Plan Assets) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 239.5 | $ 192.4
Weighted average asset allocations | 100.00% | 100.00%
Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 37.4
Level 2 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0.5
Cash [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 1.6 | 10.0
Cash [Member] | Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 1.6 | $ 10.0
Equity Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted average asset allocations | 63.00% | 60.00%
Target allocation | 65.00% |
Indexed Mutual Funds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 8.9
Indexed Mutual Funds [Member] | Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 8.9
International Mutual Funds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 15.5 | 3.0
International Mutual Funds [Member] | Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 15.5 | $ 3.0
Debt Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted average asset allocations | 37.00% | 30.00%
Target allocation | 35.00% |
Other [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted average asset allocations | 0.00% | 10.00%
Target allocation | 0.00% |
Mutual Funds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 17.5 | $ 9.2
Mutual Funds [Member] | Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | 17.5 | 9.2
Insurance Contract [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0.5
Insurance Contract [Member] | Level 2 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 0.5
Balanced Mutual Fund [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 6.3
Balanced Mutual Fund [Member] | Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | 6.3
Pension Plan Assets In The Fair Value Hierarchy [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | 34.6 | 37.9
Pension Plan Assets In The Fair Value Hierarchy [Member] | Level 1 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | 34.6 |
Pension Plan Assets In The Fair Value Hierarchy [Member] | Level 2 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 |
Pension plan assets measured at NAV [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 204.9 | $ 154.5
Minimum [Member] | Equity Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Target Allocation Percentage | .6 |
Minimum [Member] | Debt Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Target Allocation Percentage | .3 |
Minimum [Member] | Other [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Target Allocation Percentage | 0 |
Maximum [Member] | Equity Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Target Allocation Percentage | .7 |
Maximum [Member] | Debt Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Target Allocation Percentage | .4 |
Maximum [Member] | Other [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined Benefit Plan, Target Allocation Percentage | .05 |

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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Description of target allocation percentages, or range of percentages, considering the major categories of plan assets as of the latest statement of financial position presented (on a weighted-average basis for employers with more than one plan) that are pertinent to an understanding of the plan investment policies and strategies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 + Details
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