Employee Benefits - Changes in the Fair Value of Plan Assets Categorized as Level 3 (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	$ 2,966.8	$ 2,709.2
Realized and unrealized gains (losses)	(122.2)	368.7
Ending balance at December 31	2,690.8	2,966.8
Pension Plan \| Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	100.9
Ending balance at December 31	0.0	100.9
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	100.1	14.6
Realized and unrealized gains (losses)	8.0	2.8
Purchases	18.3	55.5
Liquidations	(1.7)
Transfers into (out of) level 3	(124.7)	27.2
Ending balance at December 31	0.0	100.1
Pension Plan \| Level 3 \| International Equity
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)	(0.1)	(0.2)
Purchases	0.0	1.0
Liquidations	0.0
Transfers into (out of) level 3	(0.7)	0.0
Ending balance at December 31	0.0	0.8
Pension Plan \| Level 3 \| U.S. bonds
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.0	0.8
Realized and unrealized gains (losses)		(0.8)
Purchases		0.0
Transfers into (out of) level 3		0.0
Ending balance at December 31	0.0	0.0
OPEB Plan
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	841.5	773.5
Realized and unrealized gains (losses)	(35.2)	95.9
Ending balance at December 31	771.7	841.5
OPEB Plan \| Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	7.9
Ending balance at December 31	0.0	7.9
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	7.7	1.3
Realized and unrealized gains (losses)	1.1	0.3
Purchases	1.5	3.6
Liquidations	(0.2)
Transfers into (out of) level 3	(10.1)	2.5
Ending balance at December 31	0.0	7.7
OPEB Plan \| Level 3 \| International Equity
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.2	0.0
Realized and unrealized gains (losses)	0.0	0.0
Purchases	0.0	0.2
Liquidations	0.0
Transfers into (out of) level 3	(0.2)	0.0
Ending balance at December 31	0.0	0.2
OPEB Plan \| Level 3 \| U.S. bonds
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.0
Ending balance at December 31	$ 0.0	$ 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.

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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

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
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-Publisher FASB
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- Definition

Amount, measured using unobservable input, of decrease in plan asset of defined benefit plan from settlement of trade associated with underlying investment.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

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- Definition

Amount, measured using unobservable input, of increase in plan asset of defined benefit plan from purchase.

+ References

+ 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

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