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

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.

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

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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.

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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

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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

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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

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