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Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 503.0 | $ 461.1
Investment funds |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 14.7 | 14.4
Limited partnerships |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 29.5 | 28.0
Fixed income securities |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 23.4 | 22.9
Insurance contracts |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 7.9 | 7.8
Other plan assets |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 27.7 | 20.1
Total plan assets subject to leveling |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 103.2 | 93.2
Investments measured at net asset value |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 399.8 | 367.9
Level 1 | Investment funds |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 14.7 | 14.4
Level 1 | Limited partnerships |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 1 | Fixed income securities |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 23.4 | 22.6
Level 1 | Insurance contracts |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 1 | Other plan assets |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 27.7 | 20.1
Level 1 | Total plan assets subject to leveling |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 65.8 | 57.1
Level 2 | Investment funds |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 2 | Limited partnerships |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 2 | Fixed income securities |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.3
Level 2 | Insurance contracts |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 7.9 | 7.8
Level 2 | Other plan assets |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 2 | Total plan assets subject to leveling |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 7.9 | 8.1
Level 3 | Investment funds |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 3 | Limited partnerships |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 29.5 | 28.0
Balance at beginning of period | 28.0 | 31.0
Actual return on plan assets | 1.5 | (3.0)
Balance at end of period | 29.5 | 28.0
Level 3 | Fixed income securities |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 3 | Insurance contracts |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 3 | Other plan assets |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0
Level 3 | Total plan assets subject to leveling |  |
Fair value assets and liabilities measured on recurring basis |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 29.5 | $ 28.0

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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held. + 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)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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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