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	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Apr. 01, 2017	Sep. 30, 2017	Sep. 30, 2018
Successor
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses	$ 1,047.9	$ 1,039.1		$ 1,967.0	$ 3,051.6
Selling and administrative expenses	38.6	33.7		68.4	119.7
Operating profit	130.3	209.5		362.1	534.8
Net periodic benefit costs, excluding service cost	(4.5)	(6.6)		(13.2)	(13.6)
Income from continuing operations before income taxes	$ 97.7	149.6		255.7	443.5
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)		(13.8)
Successor | Selling and administrative expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07		0.3		0.6
Successor | Operating profit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07		6.6		13.2
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		13.2
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		0.0
Successor | Before Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses		1,046.0		1,980.8
Selling and administrative expenses		33.4		67.8
Operating profit		202.9		348.9
Net periodic benefit costs, excluding service cost		0.0		0.0
Income from continuing operations before income taxes		$ 149.6		$ 255.7
Predecessor
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating costs and expenses			$ 950.2
Selling and administrative expenses			36.3
Operating profit			212.5
Net periodic benefit costs, excluding service cost			(14.4)
Income from continuing operations before income taxes			(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)