Accumulated Other Comprehensive Income - Reclassification from AOCI (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
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost			$ (946.5)	$ (927.9)		$ (2,003.7)
Net periodic benefit costs, excluding service cost			4.6	6.6		9.1
Net periodic benefit costs, excluding service cost			(44.1)	(34.7)		(81.1)
Income tax benefit (provision)			(7.4)	(4.7)		(17.5)
Total after income taxes			120.0	101.4		328.3
Successor | Defined benefit pension plans
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost			$ (2.3)	$ (1.3)		$ (4.6)
Predecessor
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost		$ 0.0			$ (950.2)
Net periodic benefit costs, excluding service cost		0.0			14.4
Net periodic benefit costs, excluding service cost		0.0			(36.3)
Income tax benefit (provision)		266.0			263.8
Total after income taxes		$ (319.8)			(195.5)
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Net actuarial loss associated with postretirement plans and workers' compensation obligations
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Insignificant items	[1]				2.7
Total before income taxes	[1]				(9.1)
Income tax benefit (provision)	[1]				3.3
Total after income taxes	[1]				(5.8)
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Prior service credit associated with postretirement plans
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before income taxes	[1]				2.2
Income tax benefit (provision)	[1]				(0.8)
Total after income taxes	[1]				1.4
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Cash flow hedges
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before income taxes					(27.7)
Income tax benefit (provision)	[1]				9.1
Total after income taxes					(18.6)
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Foreign currency cash flow hedge contracts | Cash flow hedges
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost	[1]				(16.6)
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Fuel and explosives commodity swaps | Cash flow hedges
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost	[1]				(11.0)
Predecessor | Reclassification out of Accumulated Other Comprehensive Income | Insignificant items | Cash flow hedges
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Insignificant items	[1]				(0.1)
Predecessor | Postretirement health care and life insurance benefits | Reclassification out of Accumulated Other Comprehensive Income | Net actuarial loss associated with postretirement plans and workers' compensation obligations
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost	[1]				5.5
Predecessor | Postretirement health care and life insurance benefits | Reclassification out of Accumulated Other Comprehensive Income | Prior service credit associated with postretirement plans
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost	[1]				(2.3)
Predecessor | Defined benefit pension plans
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost					5.7
Predecessor | Defined benefit pension plans | Reclassification out of Accumulated Other Comprehensive Income | Net actuarial loss associated with postretirement plans and workers' compensation obligations
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost	[1]				6.3
Predecessor | Defined benefit pension plans | Reclassification out of Accumulated Other Comprehensive Income | Prior service credit associated with postretirement plans
Reclassification Adjustment Out of Accumulated Other Comprehensive Income (Loss) [Line Items]
Net periodic benefit costs, excluding service cost	[1]				$ 0.1

[1] Presented as gains (losses) in the unaudited condensed consolidated statements of operations.