Newly Adopted Accounting Standards and Accounting Standards Not Yet Implemented - Adjustments Due to ASU 2017-07 (Details) - Predecessor $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)
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	126.5
Operating costs and expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07	(13.5)
Selling and administrative expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07	(0.9)
Operating profit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustments due to ASU 2017-07	14.4
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
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
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	$ 126.5