Newly Adopted Accounting Standards and Accounting Standards Not Yet Implemented - Adjustments Due to ASU 2017-07 (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
	Apr. 01, 2017	Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2018
Successor
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses		$ 946.5	$ 927.9		$ 2,003.7
Selling and administrative expenses		44.1	34.7		81.1
Operating profit		165.3	152.6		404.5
Net periodic benefit costs, excluding service cost		(4.6)	(6.6)		(9.1)
Income from continuing operations before income taxes		$ 127.4	106.1		345.8
Adjustments due to ASU 2017-07					$ (22.5)
Successor | Operating costs and expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07			(6.9)
Successor | Selling and administrative expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07			0.3
Successor | Operating profit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07			6.6
Successor | Net periodic benefit costs, excluding service cost
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07			6.6
Successor | Income from continuing operations before income taxes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07			0.0
Successor | Before Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses			934.8
Selling and administrative expenses			34.4
Operating profit			146.0
Net periodic benefit costs, excluding service cost			0.0
Income from continuing operations before income taxes			$ 106.1
Predecessor
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses	$ 0.0			$ 950.2
Selling and administrative expenses	0.0			36.3
Operating profit	0.0			212.5
Net periodic benefit costs, excluding service cost	0.0			(14.4)
Income from continuing operations before income taxes	$ (585.8)			(459.3)
Predecessor | Operating costs and expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07				(13.5)
Predecessor | Selling and administrative expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07				(0.9)
Predecessor | Operating profit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07				14.4
Predecessor | Net periodic benefit costs, excluding service cost
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07				14.4
Predecessor | Income from continuing operations before income taxes
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07				0.0
Predecessor | Before Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses				963.7
Selling and administrative expenses				37.2
Operating profit				198.1
Net periodic benefit costs, excluding service cost				0.0
Income from continuing operations before income taxes				$ (459.3)