Retirement Benefits - Plan Asssets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 177.9	$ 181.7	$ 219.2
% 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	$ 0.8	$ 0.0
% 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%	16.00%
% Target Allocation	19.00%	19.00%
UMWA Plans | U.S. large-cap(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 32.8	$ 29.1
UMWA Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	8.00%	7.00%
% 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	$ 13.8	$ 12.0
UMWA Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	23.00%	20.00%
% Target Allocation	24.00%	24.00%
UMWA Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 40.4	$ 35.8
UMWA Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	4.00%	3.00%
% Target Allocation	4.00%	4.00%
UMWA Plans | Emerging markets(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 6.9	$ 6.2
UMWA Plans | Dynamic asset allocation(c)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	7.00%	6.00%
% Target Allocation	7.00%	7.00%
UMWA Plans | Dynamic asset allocation(c) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 12.1	$ 10.8
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	$ 3.5	$ 3.3
UMWA Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	4.00%	3.00%
% Target Allocation	4.00%	4.00%
UMWA Plans | Emerging markets(f) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 6.9	$ 6.1
UMWA Plans | Multi asset real return(i)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	4.00%
% Target Allocation	5.00%	5.00%
UMWA Plans | Multi asset real return(i) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 8.6	$ 7.6
UMWA Plans | Core property(g) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	11.00%	14.00%
% Target Allocation	10.00%	10.00%
UMWA Plans | Core property(g) (l) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 20.3	$ 25.2
UMWA Plans | Structured credit(h) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	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	$ 9.6	$ 13.4
UMWA Plans | Global private equity(j) (l)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 14.8	$ 15.5
% Actual Allocation	8.00%	9.00%
% Target Allocation	7.00%	7.00%
UMWA Plans | Energy debt(k) (l)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 7.4	$ 16.7
% Actual Allocation	4.00%	9.00%
% 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	$ 699.3	$ 686.6	777.2
% 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	$ 3.8	$ 4.1
% 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	12.00%	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	$ 83.6	$ 79.1
U.S. Plans | U.S. small/mid-cap(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	4.00%
% 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	$ 34.7	$ 28.5
U.S. Plans | International(a)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	15.00%	15.00%
% Target Allocation	15.00%	15.00%
U.S. Plans | International(a) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 106.2	$ 102.3
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	$ 14.0	$ 9.9
U.S. Plans | Dynamic asset allocation(c)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	4.00%	3.00%
% Target Allocation	4.00%	4.00%
U.S. Plans | Dynamic asset allocation(c) | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 28.3	$ 22.4
U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	48.00%	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	$ 277.3	$ 260.3
U.S. Plans | Long duration - Treasury strips(d) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 53.9	$ 68.6
U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	2.00%	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	$ 13.9	$ 10.9
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	$ 14.0	$ 10.4
U.S. Plans | Core property(g) (l)
Defined Benefit Plan Disclosure [Line Items]
% Actual Allocation	5.00%	7.00%
% Target Allocation	5.00%	5.00%
U.S. Plans | Core property(g) (l) | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 36.3	$ 44.7
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	$ 33.3	$ 45.4
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 215.1	$ 180.6	$ 202.9
% 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	$ 177.9	$ 163.4
Non-U.S. 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%
Non-U.S. Plans | Equity securities:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 105.2	$ 86.3
% Actual Allocation	49.00%	48.00%
% Target Allocation	50.00%	52.00%
Non-U.S. Plans | U.S. equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 30.2	$ 23.8
Non-U.S. Plans | Canadian equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	41.4	32.5
Non-U.S. Plans | European equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	3.6	4.0
Non-U.S. Plans | Emerging markets(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	5.0	4.8
Non-U.S. Plans | Other global equity funds(a)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	25.0	21.2
Non-U.S. Plans | Fixed-income securities:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 92.0	$ 91.8
% Actual Allocation	43.00%	51.00%
% Target Allocation	41.00%	47.00%
Non-U.S. Plans | Long duration - mutual fund(d)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 79.5	$ 70.4
Non-U.S. Plans | High yield(e)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.6	1.2
Non-U.S. Plans | Emerging markets(f)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	1.8	1.5
Non-U.S. Plans | European fixed-income funds(b)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	9.1	18.7
Non-U.S. Plans | Other types of investments:
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 17.4	$ 1.7
% Actual Allocation	8.00%	1.00%
% Target Allocation	9.00%	1.00%
Non-U.S. Plans | Property funds(f)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 8.1
Non-U.S. Plans | Global infrastructure fund(g)
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	7.5
Non-U.S. Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value	$ 1.8	$ 1.7