Retirement Benefits - Plan Asssets (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) hedge_fund	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Maximum average weighted maturity (in years)	10 years
Number of separate hedge-fund investments | hedge_fund	30
Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Portfolio duration (in years)	10 years
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 721.4	$ 786.3	$ 811.8
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
Postretirement cost:	4.10%	5.00%	4.20%
Benefit obligation at year end:	4.50%	4.10%	5.00%
Expected return on assets	7.50%	8.00%	8.00%
U.S. Plans | Cash, cash equivalents and receivables
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 4.2	$ 4.1
% Actual Allocation	1.00%	1.00%
% Target Allocation	0.00%	0.00%
U.S. Plans | U.S. large-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	11.00%	12.00%
% Target Allocation	12.00%	12.00%
U.S. Plans | U.S. large-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 80.1	$ 93.1
U.S. Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	4.00%	5.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | U.S. small/mid-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 30.0	$ 40.2
U.S. Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	9.00%	9.00%
% Target Allocation	10.00%	10.00%
U.S. Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 66.1	$ 72.6
U.S. Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	1.00%	2.00%
% Target Allocation	2.00%	2.00%
U.S. Plans | Emerging markets(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 8.6	$ 13.8
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	$ 23.9	$ 33.7
U.S. Plans | U.S. managed volatility equities(n)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	0.00%	0.00%
% Target Allocation	5.00%	0.00%
U.S. Plans | U.S. managed volatility equities(n) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.0	$ 0.0
U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	50.00%	47.00%
% 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	$ 290.0	$ 277.6
U.S. Plans | Long duration - Treasury strips(d) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 71.7	$ 95.7
U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	1.00%	2.00%
% Target Allocation	2.00%	2.00%
U.S. Plans | High yield(e) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 9.0	$ 15.3
U.S. Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	1.00%	2.00%
% Target Allocation	2.00%	2.00%
U.S. Plans | Emerging markets(f) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 9.0	$ 14.5
U.S. Plans | Hedge fund of funds(g) (m)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	5.00%
% Target Allocation	0.00%	5.00%
U.S. Plans | Hedge fund of funds(g) (m) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 36.6	$ 37.9
U.S. Plans | Core property(h) (m)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	7.00%	6.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | Core property(h) (m) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 51.5	$ 45.0
U.S. Plans | Structured credit(i) (m)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	6.00%	5.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | Structured credit(i) (m) | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 40.7	$ 42.8
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 164.8	$ 179.3	$ 322.0
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
Postretirement cost:	5.10%	6.30%	5.30%
Benefit obligation at year end:	5.10%	5.10%	6.30%
Expected return on assets	5.58%	5.83%	4.64%
Non-U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 149.1	$ 177.9
Non-U.S. Plans | Cash, cash equivalents and receivables
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.8	$ 1.1
% 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	$ 86.1	$ 105.6
% Actual Allocation	52.00%	59.00%
% Target Allocation	60.00%	65.00%
Non-U.S. Plans | U.S. equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 24.5	$ 31.6
Non-U.S. Plans | Canadian equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	28.6	39.6
Non-U.S. Plans | European equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	9.5	8.8
Non-U.S. Plans | Asia Pacific equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.8	1.7
Non-U.S. Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	4.5	3.5
Non-U.S. Plans | Other non-U.S. equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	17.2	20.4
Non-U.S. Plans | Fixed-income securities:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 76.6	$ 71.0
% Actual Allocation	47.00%	40.00%
% Target Allocation	38.00%	35.00%
Non-U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 60.8	$ 27.9
Non-U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.2	1.0
Non-U.S. Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.2	1.2
Non-U.S. Plans | Global credit(b)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	0.4	0.3
Non-U.S. Plans | Canadian fixed-income funds(c)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	0.0	25.7
Non-U.S. Plans | European fixed-income funds(d)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	13.0	14.9
Non-U.S. Plans | Other types of investments:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 1.3	$ 1.6
% Actual Allocation	1.00%	1.00%
% Target Allocation	2.00%	0.00%
Non-U.S. Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 1.3	$ 1.6
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 227.4	$ 264.6	$ 284.4
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
Postretirement cost:	4.00%	4.70%	3.90%
Benefit obligation at year end:	4.40%	4.00%	4.70%
UMWA Plans | Cash, cash equivalents and receivables
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 1.6	$ 0.0
% Actual Allocation	1.00%	0.00%
% Target Allocation	0.00%	0.00%
UMWA Plans | U.S. large-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	21.00%	21.00%
% Target Allocation	22.00%	21.00%
UMWA Plans | U.S. large-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 47.5	$ 58.5
UMWA Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	9.00%	10.00%
% Target Allocation	10.00%	9.00%
UMWA Plans | U.S. small/mid-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 20.2	$ 25.5
UMWA Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	18.00%	19.00%
% Target Allocation	19.00%	18.00%
UMWA Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 40.5	$ 49.3
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	$ 9.0	$ 10.7
UMWA Plans | Dynamic asset allocation(c)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	7.00%	8.00%
% Target Allocation	7.00%	7.00%
UMWA Plans | Dynamic asset allocation(c) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 15.9	$ 20.0
UMWA Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	3.00%	4.00%
% Target Allocation	2.00%	4.00%
UMWA Plans | High yield(e) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 6.9	$ 10.9
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	$ 9.0	$ 10.4
UMWA Plans | Multi asset real return(j)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	6.00%	8.00%
% Target Allocation	5.00%	8.00%
UMWA Plans | Multi asset real return(j) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 14.7	$ 21.6
UMWA Plans | Hedge fund of funds(g) (m)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	3.00%	6.00%
% Target Allocation	0.00%	3.00%
UMWA Plans | Hedge fund of funds(g) (m) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 6.4	$ 14.6
UMWA Plans | Core property(h) (m)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	14.00%	11.00%
% Target Allocation	10.00%	10.00%
UMWA Plans | Core property(h) (m) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 32.9	$ 28.8
UMWA Plans | Structured credit(i) (m)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	6.00%	5.00%
% Target Allocation	5.00%	5.00%
UMWA Plans | Structured credit(i) (m) | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 13.6	$ 14.3
UMWA Plans | Global private equity(k) (m)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 3.2	$ 0.0
% Actual Allocation	1.00%	0.00%
% Target Allocation	7.00%	7.00%
UMWA Plans | Energy debt(l) (m)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 6.0	$ 0.0
% Actual Allocation	3.00%	0.00%
% Target Allocation	5.00%	0.00%
Black Lung and Other Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.0	$ 0.0	$ 0.0
Postretirement cost:	3.70%	4.40%	3.50%
Benefit obligation at year end:	4.20%	3.70%	4.40%
Retirement benefits other than pension
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 227.4	$ 264.6	$ 284.4
Postretirement cost:	4.10%	4.70%	3.90%
Benefit obligation at year end:	4.40%	4.10%	4.70%
Expected return on assets	8.25%	8.25%	8.25%
Total
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 886.2	$ 965.6	$ 1,133.8