Retirement Benefits (Fair Value Disclosures) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	$ 11,844.5	$ 10,179.7	$ 9,995.6
Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	2,372.4	1,961.2	$ 1,943.7
U.S. | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	466.2	402.4
U.S. | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	43.0	38.7
International | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	2,934.2	2,285.6
International | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	182.5	146.3
Developed markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	3,182.9	2,631.3
Developed markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	71.2	68.0
Developed markets - repurchase agreements | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	(1,372.9)	(1,024.4)
Emerging markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	584.7	450.0
Emerging markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	53.1	42.6
Hedge funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	2,984.6	2,904.6
Hedge funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	256.0	261.0
Equity-like funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,639.6	1,355.0
Equity-like funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	137.0	116.0
Cash value of trust owned insurance contract | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,524.6	1,208.3
Real estate | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	563.9	504.1
Real estate | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	33.0	34.8
Other | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	861.3	671.1
Other | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	72.0	45.5
Significant Unobservable Inputs (Level 3) | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	22.1	17.1
Significant Unobservable Inputs (Level 3) | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	2.1	1.7
Significant Unobservable Inputs (Level 3) | U.S. | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | U.S. | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | International | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | International | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Developed markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Developed markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Developed markets - repurchase agreements | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Emerging markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	3.1	0.3
Significant Unobservable Inputs (Level 3) | Emerging markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.3	0.0
Significant Unobservable Inputs (Level 3) | Hedge funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Hedge funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Equity-like funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	16.8	16.8
Significant Unobservable Inputs (Level 3) | Equity-like funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1.6	1.7
Significant Unobservable Inputs (Level 3) | Cash value of trust owned insurance contract | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Real estate | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Real estate | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Unobservable Inputs (Level 3) | Other | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	2.2	0.0
Significant Unobservable Inputs (Level 3) | Other | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.2	0.0
Significant Other Observable Inputs (Level 2) | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,950.1	1,247.0
Significant Other Observable Inputs (Level 2) | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,663.0	1,288.6
Significant Other Observable Inputs (Level 2) | U.S. | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | U.S. | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | International | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | International | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Developed markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	2,468.2	1,983.0
Significant Other Observable Inputs (Level 2) | Developed markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	63.5	58.4
Significant Other Observable Inputs (Level 2) | Developed markets - repurchase agreements | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	(1,372.9)	(1,024.4)
Significant Other Observable Inputs (Level 2) | Emerging markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	252.0	180.1
Significant Other Observable Inputs (Level 2) | Emerging markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	24.4	18.2
Significant Other Observable Inputs (Level 2) | Hedge funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Hedge funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Equity-like funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.2
Significant Other Observable Inputs (Level 2) | Equity-like funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Cash value of trust owned insurance contract | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,524.6	1,208.3
Significant Other Observable Inputs (Level 2) | Real estate | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Real estate | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Significant Other Observable Inputs (Level 2) | Other | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	602.8	108.1
Significant Other Observable Inputs (Level 2) | Other | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	50.5	3.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,645.4	1,672.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	128.7	131.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	199.6	165.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	19.4	16.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | International | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	955.1	770.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | International | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	61.3	52.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Developed markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	28.7	27.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Developed markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Developed markets - repurchase agreements | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Emerging markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	4.2	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Emerging markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Hedge funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Hedge funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity-like funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity-like funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash value of trust owned insurance contract | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	338.6	344.5
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	33.0	34.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	119.2	365.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	15.0	28.1
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	8,226.9	7,242.9
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	578.6	539.3
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | U.S. | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	266.6	236.9
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | U.S. | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	23.6	22.0
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | International | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,979.1	1,515.1
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | International | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	121.2	94.3
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Developed markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	686.0	621.1
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Developed markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	7.7	9.6
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Developed markets - repurchase agreements | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Emerging markets | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	325.4	269.6
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Emerging markets | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	28.4	24.4
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Hedge funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	2,984.6	2,904.6
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Hedge funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	256.0	261.0
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Equity-like funds | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	1,622.8	1,338.0
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Equity-like funds | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	135.4	114.3
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Cash value of trust owned insurance contract | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Real estate | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	225.3	159.6
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Real estate | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	0.0	0.0
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Other | Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	137.1	198.0
Fair Value, Retirement Benefits, Investments Valued at Net Asset Value | Other | Retiree Health Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Line Items] [Line Items]
Fair value of plan assets	$ 6.3	$ 13.7