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Employee Benefits - Changes in the Fair Value of Plan Assets Categorized as Level 3 (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Pension Plan |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | $ 2,709.2 | $ 2,755.1
Realized and unrealized gains (losses) | 368.7 | 199.4
Ending balance at December 31 | 2,966.8 | 2,709.2
Pension Plan | Level 3 | Private equity and real estate |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 14.6 | 5.5
Realized and unrealized gains (losses) | 2.8 | 0.5
Purchases | 55.5 | 8.6
Transfers into level 3 | 27.2 |
Ending balance at December 31 | 100.1 | 14.6
Pension Plan | Level 3 | International equity |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.0 |
Realized and unrealized gains (losses) | (0.2) |
Purchases | 1.0 |
Transfers into level 3 | 0.0 |
Ending balance at December 31 | 0.8 | 0.0
Pension Plan | Level 3 | US bonds |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.8 | 0.0
Realized and unrealized gains (losses) | (0.8) | 0.0
Purchases | 0.0 | 0.8
Transfers into level 3 | 0.0 |
Ending balance at December 31 | 0.0 | 0.8
OPEB Plan |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 773.5 | 749.8
Realized and unrealized gains (losses) | 95.9 | 51.5
Ending balance at December 31 | 841.5 | 773.5
OPEB Plan | Level 3 | Private equity and real estate |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 1.3 | 0.4
Realized and unrealized gains (losses) | 0.3 | 0.1
Purchases | 3.6 | 0.8
Transfers into level 3 | 2.5 |
Ending balance at December 31 | 7.7 | 1.3
OPEB Plan | Level 3 | International equity |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.0 |
Realized and unrealized gains (losses) | 0.0 |
Purchases | 0.2 |
Transfers into level 3 | 0.0 |
Ending balance at December 31 | $ 0.2 | $ 0.0

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + 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)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + 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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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from purchases, sales and settlements. + 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)(B) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan, for assets transferred into (out of) Level 3 of fair value hierarchy. Includes, but is not limited to, transfer due to change in observability of significant inputs. + 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)(C) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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