Retirement Benefits - Narrative (Details) $ in Millions		1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Apr. 30, 2012	Dec. 31, 2014 USD ($) beneficiary participant	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) beneficiary participant	Dec. 31, 2013 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Benefit plan experience gains (losses)					$ 4.4	$ (134.3)	$ 265.0
Estimated contributions					6.3
Discount percent							50.00%
Multiemployer pension expense					0.1	0.1	$ 0.2
Defined contribution plan maximum annual contribution per employee percent	1.00%	1.50%
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			$ 904.1		844.8	$ 904.1
Number of participants | participant			4,300			4,300
Pension plan beneficiaries | beneficiary			15,200			15,200
Settlement loss			$ 56.1		0.0	$ 56.1	0.1
Benefit plan experience gains (losses)						40.0
UMWA obligation increase due to Cadillac plan excise tax					(24.5)	(101.1)
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			$ 242.9		226.5	242.9
Settlement loss					5.7	6.3	2.6
Estimated contributions					11.4
UMWA obligation increase due to Cadillac plan excise tax					$ (20.0)	$ (50.3)
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Black lung health care cost trend rate					7.00%	7.00%
Black lung health care cost ultimate rate					5.00%
UMWA obligation increase due to Cadillac plan excise tax					$ (11.7)	$ (58.6)
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Black lung health care cost trend rate						5.00%
UMWA obligation increase due to Cadillac plan excise tax					0.1	$ (23.2)
Pension plan
Defined Benefit Plan Disclosure [Line Items]
Experience loss					28.9
Prior service cost					(0.1)
Settlement loss					5.7	62.4	$ 2.7
UMWA obligation increase due to Cadillac plan excise tax					(44.5)	(151.4)
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
UMWA obligation increase due to Cadillac plan excise tax					(11.6)	(81.8)
Nonqualified U.S. pension plan
Defined Benefit Plan Disclosure [Line Items]
Estimated contributions					$ 0.6
UMWA Excise Tax
Defined Benefit Plan Disclosure [Line Items]
Excise tax percent					40.00%
UMWA obligation increase due to Cadillac plan excise tax					$ 15.1	$ 24.8
Canada Plan
Defined Benefit Plan Disclosure [Line Items]
Black lung health care cost trend rate					7.00%
Black lung health care cost ultimate rate					5.00%
Brazil Plan
Defined Benefit Plan Disclosure [Line Items]
Black lung health care cost trend rate					3.00%
Forecast | Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Experience loss				$ 19.4
Prior service cost				$ 2.9
Structured credit(i) (m)
Defined Benefit Plan Disclosure [Line Items]
Required notice (in days)					65 days
Structured credit(i) (m) | UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Lockup provision						2 years
Structured credit(i) (m) | Pension plan
Defined Benefit Plan Disclosure [Line Items]
Lockup provision							2 years
Energy debt(l) (m)
Defined Benefit Plan Disclosure [Line Items]
Required notice (in days)					95 days
Energy debt(l) (m) | UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Lockup provision					3 years
Hedge fund of funds
Defined Benefit Plan Disclosure [Line Items]
Required notice (in days)					95 days
Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Unfunded commitments					$ 18.0