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Pension and Savings Plans Fair Value of Assets in the Master Trust by Asset (Details) - USD ($) $ in Millions	Dec. 31, 2017	Apr. 01, 2017	Dec. 31, 2016	Dec. 31, 2015
Predecessor
Defined Benefit Plan Disclosure [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			$ 295.1
Predecessor | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			162.8
Predecessor | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			301.0
Predecessor | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period		$ 13.8	14.1	$ 23.0
Predecessor | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			119.9
Predecessor | Mutual funds | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			119.9
Predecessor | Mutual funds | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Mutual funds | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | International equity securities | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			47.8
Predecessor | U.S. government securities | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			25.1
Predecessor | U.S. government securities | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			22.7
Predecessor | International government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			12.6
Predecessor | International government securities | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | International government securities | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			12.6
Predecessor | International government securities | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			109.0
Predecessor | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			265.7
Predecessor | Corporate bonds | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Corporate bonds | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			265.7
Predecessor | Corporate bonds | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			17.8
Predecessor | Cash funds | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			17.8
Predecessor | Cash funds | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Cash funds | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Real estate investment trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			14.1
Predecessor | Real estate investment trusts | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Real estate investment trusts | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			0.0
Predecessor | Real estate investment trusts | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			14.1
Predecessor | Private mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			186.1
Predecessor | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			773.0	$ 757.3
Predecessor | Fair Value Pension Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period			$ 477.9
Successor
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	$ 0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	287.9
Successor | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	146.3
Successor | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	330.6
Successor | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	11.8	13.8
Successor | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	108.0
Successor | Mutual funds | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	108.0
Successor | Mutual funds | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Mutual funds | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	29.3
Successor | U.S. government securities | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	7.5
Successor | U.S. government securities | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	21.8
Successor | U.S. government securities | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | International government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	17.7
Successor | International government securities | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | International government securities | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	17.7
Successor | International government securities | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	107.5
Successor | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	291.1
Successor | Corporate bonds | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Corporate bonds | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	291.1
Successor | Corporate bonds | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	30.8
Successor | Cash funds | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	30.8
Successor | Cash funds | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Cash funds | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Real estate investment trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	11.8
Successor | Real estate investment trusts | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Real estate investment trusts | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	0.0
Successor | Real estate investment trusts | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	11.8
Successor | Private mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	180.4
Successor | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	776.6	$ 783.1
Successor | Fair Value Pension Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of period	$ 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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+ References

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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