Retirement Benefits - Narrative (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Mar. 31, 2017	Apr. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Estimated contributions			$ 7.8
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]
UMWA obligation increase due to Cadillac plan excise tax			$ (27.2)	$ (15.8)
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
UMWA obligation increase due to Cadillac plan excise tax			$ (7.8)	(81.2)
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			$ 8.0
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			801.9	890.3
UMWA obligation increase due to Cadillac plan excise tax			(24.8)	(14.6)
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			233.9	264.8
Estimated contributions			10.4
UMWA obligation increase due to Cadillac plan excise tax			$ (2.4)	$ (1.2)
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Black lung health care cost trend rate			6.50%	6.50%
Black lung health care cost ultimate rate			5.00%	5.00%
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]
Black lung health care cost ultimate rate			5.00%
Nonqualified U.S. pension plan
Defined Benefit Plan Disclosure [Line Items]
Estimated contributions			$ 1.3
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			$ 30.5	$ 34.5
ASU 2017-07 | UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
UMWA obligation increase due to Cadillac plan excise tax			(1.4)	(66.3)
ASU 2017-07 | Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
UMWA obligation increase due to Cadillac plan excise tax			$ (6.4)	$ (14.9)