Retirement Plans & Other Postretirement Benefits - Reconciliation of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of period	$ 304.5
Fair value of plan assets, end of period	316.1	$ 304.5
Pension Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of period	304.5	296.9
Actual return on plan assets	41.9	19.7
Employer contributions	1.1	16.2
Plan participants' contributions	0.0	0.0
Benefit payments	(31.4)	(28.3)
Fair value of plan assets, end of period	316.1	304.5
Other Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of period	0.0	0.0
Actual return on plan assets	0.0	0.0
Employer contributions	3.5	3.4
Plan participants' contributions	1.2	1.1
Benefit payments	(4.7)	(4.5)
Fair value of plan assets, end of period	$ 0.0	0.0
Equity Securities \| Pension Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target percentage of investments in equity instruments (in hundredths)	60.00%
Debt Securities \| Pension Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target percentage of investments in equity instruments (in hundredths)	35.00%
Real estate, commodity & other funds
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value, beginning of period	$ 10.4
Fair value of plan assets, end of period	$ 10.7	$ 10.4
Real estate, commodity & other funds \| Pension Benefits
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Target percentage of investments in equity instruments (in hundredths)	5.00%

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 contribution received by defined benefit plan from employer which increases plan assets.

+ 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)(ii)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 4: 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)(01)
-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

+ Details

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- Definition

Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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-Section 50
-Paragraph 1
-Subparagraph (b)(5)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of contributions received by defined benefit plan from participant which increases plan assets.

+ References

+ Details

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

+ Details

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