Postretirement Health Care and Life Insurance Benefits Pre-Tax Amounts Recorded in Accumulated Other Comprehensive Loss (Details) - Predecessor - Postretirement Health Care and Life Insurance Benefits [Member] $ in Millions	3 Months Ended	12 Months Ended
	Apr. 01, 2017 USD ($)	Dec. 31, 2016 USD ($) years	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss arising during period	$ 0.0	$ 32.3	$ (35.1)
Amortization:
Actuarial loss	(5.5)	(20.4)	(24.9)
Prior service credit (cost)	2.3	9.2	6.8
Settlement related to the Patriot bankruptcy
Prior service cost	0.0	7.2	(16.6)
Total recorded in other comprehensive (income) loss	$ (3.2)	$ 28.3	$ (69.8)
Percentage of actuarial gains and losses amortized	0.00%
Defined Benefit Plan, Amortization Period | years		10.31