Accumulated Other Comprehensive Income (Loss) Items Reclassified Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									$ (950.2)	$ (3,045.1)	$ (3,536.6)	$ (4,071.4)
Income Tax Benefit (Expense)									263.8	161.0	(46.0)	(18.4)
(Loss) income from continuing operations, net of income taxes	$ (290.2)	$ (74.3)	$ 42.9	$ 133.3	$ 233.5	$ 83.9	$ 120.0	$ 208.3	(195.5)	$ 713.1	(188.3)	$ 645.7
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									2.7		0.0
Total before income taxes									(9.1)		0.0
Income Tax Benefit (Expense)									3.3		0.0
(Loss) income from continuing operations, net of income taxes									(5.8)		0.0
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement health care and life insurance benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									(5.5)		0.0
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Plans, Defined Benefit
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									(6.3)		0.0
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total before income taxes									2.2		8.7
Income Tax Benefit (Expense)									(0.8)		0.0
(Loss) income from continuing operations, net of income taxes									1.4		8.7
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Postretirement health care and life insurance benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									2.3		8.7
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension Plans, Defined Benefit
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									(0.1)		0.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Total before income taxes									(27.7)		0.0
Income Tax Benefit (Expense)									9.1		0.0
(Loss) income from continuing operations, net of income taxes									(18.6)		0.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Fuel and explosives commodity swaps | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									(11.0)		0.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Foreign currency cash flow hedge contracts | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									(16.6)		0.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Insignificant items | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Net periodic benefit costs, excluding service cost/operating expenses									$ (0.1)		$ 0.0