Retirement Benefits - Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Maximum average weighted maturity (in years)	10 years
UMWA Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 168.0	$ 177.9	$ 181.7
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
UMWA Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.5	$ 0.8
% Actual Allocation	0.00%	0.00%
% Target Allocation	0.00%	0.00%
UMWA Plans | U.S. large-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	19.00%	19.00%
% Target Allocation	19.00%	19.00%
UMWA Plans | U.S. large-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 32.2	$ 32.8
UMWA Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	8.00%	8.00%
% Target Allocation	8.00%	8.00%
UMWA Plans | U.S. small/mid-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 13.3	$ 13.8
UMWA Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	24.00%	23.00%
% Target Allocation	24.00%	24.00%
UMWA Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 40.2	$ 40.4
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	$ 7.0	$ 6.9
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	$ 10.9	$ 12.1
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	$ 3.4	$ 3.5
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	$ 6.8	$ 6.9
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	$ 8.3	$ 8.6
UMWA Plans | Core property(g) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	9.00%	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	$ 14.1	$ 20.3
UMWA Plans | Structured credit(h) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	6.00%	5.00%
% Target Allocation	5.00%	5.00%
UMWA Plans | Structured credit(h) (l) | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 10.2	$ 9.6
UMWA Plans | Global private equity(j) (l)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 13.9	$ 14.8
% Actual Allocation	8.00%	8.00%
% Target Allocation	7.00%	7.00%
UMWA Plans | Energy debt(k) (l)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 7.2	$ 7.4
% Actual Allocation	4.00%	4.00%
% Target Allocation	5.00%	5.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Maximum average weighted maturity (in years)	5 years
Minimum | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Portfolio duration (in years)	10 years
Maximum
Defined Benefit Plan Disclosure [Line Items]
Maximum average weighted maturity (in years)	7 years
Maximum | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Portfolio duration (in years)	15 years
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 747.1	$ 699.3	686.6
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
U.S. Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 3.7	$ 3.8
% Actual Allocation	0.00%	0.00%
% Target Allocation	0.00%	0.00%
U.S. Plans | U.S. large-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	16.00%	12.00%
% Target Allocation	15.00%	12.00%
U.S. Plans | U.S. large-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 117.3	$ 83.6
U.S. Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	6.00%	5.00%
% Target Allocation	6.00%	5.00%
U.S. Plans | U.S. small/mid-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 46.7	$ 34.7
U.S. Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	16.00%	15.00%
% Target Allocation	15.00%	15.00%
U.S. Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 117.8	$ 106.2
U.S. Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	2.00%	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	$ 15.6	$ 14.0
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	$ 31.4	$ 28.3
U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	46.00%	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	$ 292.8	$ 277.3
U.S. Plans | Long duration - Treasury strips(d) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 49.6	$ 53.9
U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	0.00%	2.00%
% Target Allocation	0.00%	2.00%
U.S. Plans | High yield(e) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.0	$ 13.9
U.S. Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	0.00%	2.00%
% Target Allocation	0.00%	2.00%
U.S. Plans | Emerging markets(f) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.0	$ 14.0
U.S. Plans | Core property(g) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	5.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | Core property(g) (l) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 37.0	$ 36.3
U.S. Plans | Structured credit(h) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	5.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | Structured credit(h) (l) | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 35.2	$ 33.3
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 355.8	$ 215.1	$ 180.6
% Actual Allocation	100.00%	100.00%
% Target Allocation	100.00%	100.00%
Non-U.S. Plans | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 186.0	$ 177.9
Non-U.S. Plans | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	10.3
Non-U.S. Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 0.5	$ 0.5
% 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	$ 117.4	$ 105.2
% Actual Allocation	33.00%	49.00%
% Target Allocation	32.00%	50.00%
Non-U.S. Plans | U.S. equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 32.6	$ 30.2
Non-U.S. Plans | Canadian equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	44.1	41.4
Non-U.S. Plans | European equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	3.7	3.6
Non-U.S. Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	5.6	5.0
Non-U.S. Plans | Other global equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	31.4	25.0
Non-U.S. Plans | Fixed-income securities:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 99.8	$ 92.0
% Actual Allocation	28.00%	43.00%
% Target Allocation	27.00%	41.00%
Non-U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 77.3	$ 79.5
Non-U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.8	1.6
Non-U.S. Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	2.1	1.8
Non-U.S. Plans | Canadian fixed-income securities(b)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	6.2
Non-U.S. Plans | European fixed-income funds(c)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	12.4	9.1
Non-U.S. Plans | Other types of investments:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 138.1	$ 17.4
% Actual Allocation	5.00%	8.00%
% Target Allocation	6.00%	9.00%
Non-U.S. Plans | Guaranteed contract value
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 120.2
% Actual Allocation	34.00%
% Target Allocation	35.00%
Non-U.S. Plans | Property funds(h)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 8.0	$ 8.1
Non-U.S. Plans | Global infrastructure fund(i)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	7.9	7.5
Non-U.S. Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 2.0	$ 1.8