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Pension and Savings Plans Fair Value of Assets in the Master Trust by Asset (Details) - Successor - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Apr. 01, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		$ 287.9
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period		108.0
U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	$ 198.9	29.3
International government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	12.4	17.7
Commercial Paper
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	2.1
Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period		107.5
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	466.1	291.1
Cash funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	38.4	30.8
Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	6.2	11.8
Private mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	40.7	180.4
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	219.9	146.3
Level 1 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period		108.0
Level 1 | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	181.5	7.5
Level 1 | International government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 1 | Commercial Paper
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 1 | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	38.4	30.8
Level 1 | Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	498.0	330.6
Level 2 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period		0.0
Level 2 | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	17.4	21.8
Level 2 | International government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	12.4	17.7
Level 2 | Commercial Paper
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	2.1
Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	466.1	291.1
Level 2 | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 2 | Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	6.2	11.8	$ 13.8
Level 3 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period		0.0
Level 3 | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period		0.0
Level 3 | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Level 3 | International government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 3 | Commercial Paper
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Level 3 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 3 | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0	0.0
Level 3 | Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	6.2	11.8
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	764.8	776.6	$ 783.1
Fair Value Pension Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	$ 724.1	$ 488.7

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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