Retirement Benefits - Plan Asssets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Maximum average weighted maturity (in years)	10 years
Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Portfolio duration (in years)	25 years
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 219.2	$ 217.6	$ 227.4
Postretirement cost:	4.20%	4.40%	4.10%
Benefit obligation at year end:	3.70%	4.20%	4.40%
Expected return on assets	8.25%	8.25%	8.25%
Total
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 980.1	$ 901.9	$ 886.2
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 219.2	$ 217.6	$ 227.4
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
Postretirement cost:	4.10%	4.40%	4.00%
Benefit obligation at year end:	3.60%	4.10%	4.40%
UMWA Plans | U.S. large-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	18.00%	21.00%
% Target Allocation	19.00%	22.00%
UMWA Plans | U.S. large-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 40.3	$ 45.5
UMWA Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	8.00%	9.00%
% Target Allocation	8.00%	10.00%
UMWA Plans | U.S. small/mid-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 16.8	$ 19.9
UMWA Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	23.00%	19.00%
% Target Allocation	24.00%	19.00%
UMWA Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 50.6	$ 40.1
UMWA Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	4.00%	4.00%
% Target Allocation	4.00%	4.00%
UMWA Plans | Emerging markets(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 8.6	$ 8.2
UMWA Plans | Dynamic asset allocation(c)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	7.00%	7.00%
% Target Allocation	7.00%	7.00%
UMWA Plans | Dynamic asset allocation(c) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 15.2	$ 14.9
UMWA Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	2.00%	2.00%
% Target Allocation	2.00%	2.00%
UMWA Plans | High yield(e) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 4.3	$ 4.4
UMWA Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	4.00%	4.00%
% Target Allocation	4.00%	4.00%
UMWA Plans | Emerging markets(f) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 8.7	$ 8.1
UMWA Plans | Multi asset real return(i)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	5.00%
% Target Allocation	5.00%	5.00%
UMWA Plans | Multi asset real return(i) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 10.8	$ 10.6
UMWA Plans | Core property(g) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	11.00%	11.00%
% Target Allocation	10.00%	10.00%
UMWA Plans | Core property(g) (l) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 23.1	$ 24.9
UMWA Plans | Structured credit(h) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	6.00%	7.00%
% Target Allocation	5.00%	5.00%
UMWA Plans | Structured credit(h) (l) | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 12.4	$ 16.1
UMWA Plans | Global private equity(j) (l)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 11.8	$ 8.6
% Actual Allocation	5.00%	4.00%
% Target Allocation	7.00%	7.00%
UMWA Plans | Energy debt(k) (l)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 16.6	$ 16.3
% Actual Allocation	7.00%	7.00%
% Target Allocation	5.00%	5.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Maximum average weighted maturity (in years)	5 years
Maximum
Defined Benefit Plan Disclosure [Line Items]
Maximum average weighted maturity (in years)	7 years
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 777.2	$ 728.5	$ 721.4
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
Postretirement cost:	4.30%	4.50%	4.10%
Benefit obligation at year end:	3.70%	4.30%	4.50%
Expected return on assets	7.25%	7.50%	7.50%
U.S. Plans | Cash, cash equivalents and receivables
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 4.6	$ 4.0
% Actual Allocation	1.00%	0.00%
% Target Allocation	0.00%	0.00%
U.S. Plans | U.S. large-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	11.00%	13.00%
% Target Allocation	12.00%	14.00%
U.S. Plans | U.S. large-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 88.1	$ 94.1
U.S. Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	6.00%
% Target Allocation	5.00%	6.00%
U.S. Plans | U.S. small/mid-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 36.4	$ 40.6
U.S. Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	14.00%	11.00%
% Target Allocation	15.00%	12.00%
U.S. Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 111.5	$ 80.1
U.S. Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	2.00%	1.00%
% Target Allocation	2.00%	2.00%
U.S. Plans | Emerging markets(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 15.0	$ 9.9
U.S. Plans | Dynamic asset allocation(c)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	4.00%	4.00%
% Target Allocation	4.00%	4.00%
U.S. Plans | Dynamic asset allocation(c) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 29.8	$ 26.6
U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	49.00%	48.00%
% Target Allocation	48.00%	48.00%
U.S. Plans | Long duration - mutual fund(d) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 304.2	$ 282.6
U.S. Plans | Long duration - Treasury strips(d) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 74.5	$ 65.3
U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	2.00%	1.00%
% Target Allocation	2.00%	2.00%
U.S. Plans | High yield(e) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 14.8	$ 10.5
U.S. Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	2.00%	1.00%
% Target Allocation	2.00%	2.00%
U.S. Plans | Emerging markets(f) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 15.2	$ 9.7
U.S. Plans | Core property(g) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	8.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | Core property(g) (l) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 41.0	$ 56.9
U.S. Plans | Structured credit(h) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	7.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | Structured credit(h) (l) | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 42.1	$ 48.2
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 202.9	$ 173.4	$ 164.8
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
Postretirement cost:	3.70%	5.10%	5.10%
Benefit obligation at year end:	3.50%	3.70%	5.10%
Expected return on assets	5.50%	5.65%	5.58%
Non-U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 182.6	$ 156.0
Non-U.S. Plans | Cash, cash equivalents and receivables
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.3	$ 0.9
% Actual Allocation	0.00%	0.00%
% Target Allocation	0.00%	0.00%
Non-U.S. Plans | Equity securities:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 102.5	$ 91.2
% Actual Allocation	51.00%	53.00%
% Target Allocation	52.00%	53.00%
Non-U.S. Plans | U.S. equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 28.3	$ 25.5
Non-U.S. Plans | Canadian equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	39.0	33.9
Non-U.S. Plans | European equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	4.8	4.5
Non-U.S. Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	5.6	4.6
Non-U.S. Plans | Other non-U.S. equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	24.8	22.7
Non-U.S. Plans | Fixed-income securities:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 98.4	$ 79.8
% Actual Allocation	49.00%	46.00%
% Target Allocation	47.00%	46.00%
Non-U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 77.1	$ 63.1
Non-U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.3	1.2
Non-U.S. Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.6	1.4
Non-U.S. Plans | European fixed-income funds(b)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	18.4	14.1
Non-U.S. Plans | Other types of investments:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 1.7	$ 1.5
% Actual Allocation	0.00%	1.00%
% Target Allocation	1.00%	1.00%
Non-U.S. Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 1.7	$ 1.5