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Retirement Benefits - Narrative (Details) shares in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 USD ($) employee shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 | Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Projected Retirement Benefit Expense In Next 12 months |  |  | $ 88.0 |  |  |  |
Defined Contribution Plan, Cost |  |  | $ 39.9 | $ 35.5 | $ 34.5 |  |
Other Postretirement Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Discount rate | [1] |  | 3.80% | 4.35% |  |  |
Curtailment loss |  |  | $ 0.0 | $ 0.0 | $ 0.0 |  |
Employer contributions |  |  | 0.0 | 0.0 |  |  |
Projected Retirement Benefit Expense In Next 12 months |  |  | 27.0 |  |  |  |
Defined Contribution Plan, Cost |  |  | $ 1.0 | $ 1.7 |  |  |
Assumed increase in per capita cost of covered benefits in next 12 months |  |  | 6.40% |  |  |  |
Assumed ultimate health care cost trend rate |  |  | 4.50% |  |  |  |
Year that ultimate trend rate is reached |  |  | 2038 |  |  |  |
Expected long-term rate of return on assets |  |  | 4.00% | 4.00% | 4.00% |  |
Fair value of plan assets |  |  | $ 0.1 | $ 0.6 | $ 0.7 |  |
Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Discount rate | [1] |  | 3.85% | 4.45% | 4.65% |  |
Curtailment loss |  |  | $ (0.4) | $ 0.0 | $ 0.0 |  |
Employer contributions |  |  | 49.3 | 143.4 |  |  |
Projected Retirement Benefit Expense In Next 12 months |  |  | 61.0 |  |  |  |
Accumulated benefit obligation |  |  | $ 2,482.5 | $ 2,810.5 |  |  |
Expected long-term rate of return on assets |  |  | 7.75% | 7.75% | 8.00% |  |
Actual return on plan assets |  |  | (4.80%) | 16.90% | 5.30% | (1.20%) | 6.50%
Fair value of plan assets |  |  | $ 1,772.2 | $ 2,129.6 | $ 1,894.6 |  |
Defined Benefit Plan, Weighted Average Life Expectancy |  |  | 17 years |  |  |  |
Pension Benefits [Member] | Global debt securities and cash [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Unfunded commitments |  |  | $ 41.0 |  |  |  |
Pension Benefits [Member] | Private equity [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Unfunded commitments |  |  | 84.0 |  |  |  |
Fair value of plan assets |  |  | 137.1 | 137.5 |  |  |
Pension Benefits [Member] | Hedge funds [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  | 258.3 | 307.4 |  |  |
Pension Benefits [Member] | Real estate and other [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Unfunded commitments |  |  | 51.0 |  |  |  |
Fair value of plan assets |  |  | 52.7 | 93.9 |  |  |
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  | 0.0 | 0.0 |  |  |
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member] | Private equity [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  | 0.0 | 0.0 |  |  |
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member] | Hedge funds [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  | 0.0 | 0.0 |  |  |
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member] | Real estate and other [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  | 0.0 | $ 0.0 |  |  |
Pension Benefits [Member] | Forecast [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Expected long-term rate of return on assets |  | 7.60% |  |  |  |  |
Pension Benefits [Member] | MultiemployerPlan1 [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Percent of Current Year Contributions to Plan that Company Contributions Exceed |  |  |  | 5.00% |  |  |
Pension Benefits [Member] | Nonqualified and U.K. [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Estimated employer contributions in next 12 months |  |  | $ 8.0 |  |  |  |
Pension Benefits [Member] | U.S. [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Defined benefit plan, number of employees accruing benefit service | employee |  |  | 1,600 |  |  |  |
Defined Benefit Plan, Percent Of Employees accruing benefit service |  |  | 10.00% |  |  |  |
Employer contributions |  |  | $ 40.0 | $ 135.0 | $ 115.0 |  |
Estimated employer contributions in next 12 months |  |  | $ 145.0 |  |  |  |
Percentage of total assets |  |  | 90.00% |  |  |  |
Number of shares of equity securities issued by employer included in plan assets | shares |  |  | 0.0 | 3.0 |  |  |
Fair value of employer equity securities included in plan assets |  |  |  | $ 71.3 |  |  |
Voluntary lump sum cash out [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Benefits paid |  |  |  | $ 22.0 |  |  |
Small annuity buy out [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Benefits paid |  |  | $ 97.0 |  |  |  |
Small annuity buy out [Member] | Pension Benefits [Member] | U.S. [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Number of participants | employee |  |  | 3,700 |  |  |  |
Percent of U.S. employees as participants |  |  | 17.00% |  |  |  |
[1] | Other postretirement benefits expense for 2016 was initially measured at a 4.50% discount rate. A portion of the obligation was remeasured using a 4.05% discount rate as of March 1, 2016, following a new CBA.

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