Employee Benefits - Changes in the Fair Value of Plan Assets Categorized as Level 3 (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	$ 1,019.8	$ 1,134.1
Realized and unrealized gains	156.7	(31.0)
Ending balance at December 31	1,094.6	1,019.8
Pension Plan \| Level 3
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
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	0.0	55.3
Realized and unrealized gains		4.1
Purchases		9.8
Liquidations		(1.2)
Transfers out of level 3		(68.0)
Ending balance at December 31		0.0
OPEB Plan
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	201.5	220.1
Realized and unrealized gains	35.4	(5.7)
Ending balance at December 31	228.5	201.5
OPEB Plan \| Level 3
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
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	$ 0.0	3.8
Realized and unrealized gains		0.8
Purchases		0.7
Liquidations		(0.1)
Transfers out of level 3		(5.2)
Ending balance at December 31		$ 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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Balance Type:	debit
Period Type:	duration

- 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

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
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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=118255775&loc=d3e1928-114920

+ Details

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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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Balance Type:	debit
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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

+ Details

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- Details

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- Details

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- Details

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- Details

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