Retirement Benefits - Narrative (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Mar. 31, 2017	Apr. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Estimated contributions			$ 7.3
Discount percent			50.00%
Defined contribution plan maximum annual contribution per employee percent	1.50%	2.00%
Pension plan
Defined Benefit Plan Disclosure [Line Items]
Experience loss			$ 33.0
Prior service cost (credit)			0.1
Settlement loss			2.0	$ 3.1	$ 5.7
Projected benefit payments			343.5
UMWA obligation increase due to Cadillac plan excise tax			(15.8)	(21.1)
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Experience loss			26.7
Prior service cost (credit)			(3.5)
Projected benefit payments			179.2
UMWA obligation increase due to Cadillac plan excise tax			$ (81.2)	(30.4)
Structured credit(h) (l)
Defined Benefit Plan Disclosure [Line Items]
Required notice			65 days
Energy debt(k) (l)
Defined Benefit Plan Disclosure [Line Items]
Required notice			95 days
Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Required notice			95 days
Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Unfunded commitments			$ 11.0
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			890.3	845.9
Settlement loss			0.0	0.0	0.0
Projected benefit payments			256.7
UMWA obligation increase due to Cadillac plan excise tax			(14.6)	(12.1)
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			264.8	232.3
Settlement loss			2.0	3.1	$ 5.7
Estimated contributions			14.2
Projected benefit payments			86.8
UMWA obligation increase due to Cadillac plan excise tax			(1.2)	$ (9.0)
Non-U.S. Plans | Venezuela
Defined Benefit Plan Disclosure [Line Items]
Projected benefit payments			38,740,810.4
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit payments			$ 156.3
Black lung health care cost trend rate			6.50%	7.00%
Black lung health care cost ultimate rate			5.00%
UMWA obligation increase due to Cadillac plan excise tax			$ (66.3)	$ (19.2)
UMWA Plans | Structured credit(h) (l)
Defined Benefit Plan Disclosure [Line Items]
Lockup provision						2 years
UMWA Plans | Energy debt(k) (l)
Defined Benefit Plan Disclosure [Line Items]
Lockup provision					3 years
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit payments			22.9
Black lung health care cost trend rate				5.00%
UMWA obligation increase due to Cadillac plan excise tax			(14.9)	$ (11.2)
Nonqualified U.S. pension plan
Defined Benefit Plan Disclosure [Line Items]
Estimated contributions			$ 0.7
Canada Plan
Defined Benefit Plan Disclosure [Line Items]
Black lung health care cost trend rate			6.50%
Black lung health care cost ultimate rate			5.00%
Brazil Plan
Defined Benefit Plan Disclosure [Line Items]
Black lung health care cost trend rate			3.30%
UMWA Excise Tax
Defined Benefit Plan Disclosure [Line Items]
Excise tax percent			40.00%
UMWA obligation increase due to Cadillac plan excise tax			$ 34.5	$ 19.4