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Retirement Benefits - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 USD ($) employee | Dec. 31, 2018 USD ($) employee | Dec. 31, 2017 USD ($) | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Projected Retirement Benefit Expense In Next 12 months |  | $ 60.0 |  |  |  |
Defined Contribution Plan, Cost |  | $ 44.8 | $ 39.9 | $ 35.5 |  |
Other Postretirement Benefits |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Discount rate |  | 4.35% | 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 |  | 20.0 |  |  |  |
Defined Contribution Plan, Cost |  | $ 1.0 | $ 1.0 | $ 1.7 |  |
Assumed increase in per capita cost of covered benefits in next 12 months |  | 6.00% |  |  |  |
Assumed ultimate health care cost trend rate |  | 4.50% |  |  |  |
Expected long-term rate of return on assets |  | 4.00% | 4.00% | 4.00% |  |
Fair value of plan assets |  | $ 0.1 | $ 0.1 | $ 0.6 |  |
Pension Benefits [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Discount rate |  | 4.40% | 3.85% | 4.45% |  |
Curtailment loss |  | $ 0.0 | $ (0.4) | $ 0.0 |  |
Employer contributions |  | 153.4 | 49.3 |  |  |
Projected Retirement Benefit Expense In Next 12 months |  | 40.0 |  |  |  |
Accumulated benefit obligation |  | $ 2,621.1 | $ 2,482.5 |  |  |
Expected long-term rate of return on assets |  | 7.52% | 7.75% | 7.75% |  |
Actual return on plan assets |  | 15.10% | (4.80%) | 16.90% | 5.30% | (1.20%)
Fair value of plan assets |  | $ 1,902.1 | $ 1,772.2 | $ 2,129.6 |  |
Defined Benefit Plan, Weighted Average Life Expectancy |  | 18 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 |  | $ 46.0 |  |  |  |
Pension Benefits [Member] | Private equity [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Unfunded commitments |  | 91.0 |  |  |  |
Fair value of plan assets |  | 129.0 | 137.1 |  |  |
Pension Benefits [Member] | Hedge funds [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Fair value of plan assets |  | 263.9 | 258.3 |  |  |
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 |  | 37.8 | 52.7 |  |  |
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 | 7.16% |  |  |  |  |
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 |  | $ 9.0 |  |  |  |
Pension Benefits [Member] | United States |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined benefit plan, number of employees accruing benefit service | employee |  | 1,300 |  |  |  |
Defined Benefit Plan, Percent Of Employees accruing benefit service |  | 10.00% |  |  |  |
Employer contributions |  | $ 145.0 | $ 40.0 | $ 135.0 |  |
Estimated employer contributions in next 12 months |  | $ 130.0 |  |  |  |
Percentage of total assets |  | 90.00% |  |  |  |
Small annuity buy out [Member] | Pension Benefits [Member] |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Benefits paid |  | $ 96.0 | $ 97.0 |  |  |
Small annuity buy out [Member] | Pension Benefits [Member] | United States |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Number of participants | employee |  | 1,800 | 3,700 |  |  |
Percent of U.S. employees as participants |  | 10.00% | 17.00% |  |  |

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