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Retirement Benefits - Narrative (Details) - USD ($) shares in Millions, $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2017 | Jul. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Projected Retirement Benefit Expense In Next 12 months |  |  |  | $ 52.0 |  |  |  |
Other postretirement benefit costs for a defined contribution plan |  |  |  | 35.5 | $ 34.5 | $ 41.2 |  |
Other Postretirement Benefits [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Employer contributions |  |  |  | 0.0 | $ 0.0 |  |  |
Projected Retirement Benefit Expense In Next 12 months |  |  |  | 23.0 |  |  |  |
Other postretirement benefit costs for a defined contribution plan |  |  |  | $ 1.7 |  |  |  |
Assumed increase in per capita cost of covered benefits in next 12 months |  |  |  | 6.80% |  |  |  |
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 |  |  |  | $ 0.6 | $ 0.7 | $ 1.8 |  |
Other Postretirement 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 |  |  |  | 0.6 |  |  |  |
Pension Benefits [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Employer contributions |  |  |  | 143.4 | 125.1 |  |  |
Projected Retirement Benefit Expense In Next 12 months |  |  |  | 29.0 |  |  |  |
Accumulated benefit obligation |  |  |  | $ 2,810.5 | $ 2,710.7 |  |  |
Expected long-term rate of return on assets |  |  |  | 7.75% | 8.00% | 8.00% |  |
Actual return on plan assets |  |  |  | 16.90% | 5.30% | (1.20%) | 6.50% | 14.30%
Fair value of plan assets |  |  |  | $ 2,129.6 | $ 1,894.6 | $ 1,962.3 |  |
Defined Benefit Plan, Weighted Average Life Expectancy |  |  |  | 17 years |  |  |  |
Pension Benefits [Member] | Private equity [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Fair value of plan assets |  |  |  | $ 137.5 | 142.6 |  |  |
Pension Benefits [Member] | Hedge funds [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Fair value of plan assets |  |  |  | 307.4 | 283.9 |  |  |
Pension Benefits [Member] | Real estate and other [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Fair value of plan assets |  |  |  | 93.9 | 96.5 |  |  |
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 | 0.2 | $ 194.6 |  |
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 |  |  | 7.75% |  |  |  |  |
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] | U.S. [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Employer contributions | $ 135.0 | $ 115.0 |  |  |  |  |  |
Estimated employer contributions in next 12 months |  |  |  | $ 40.0 |  |  |  |
Percentage of total assets |  |  |  | 96.00% |  |  |  |
Number of shares of equity securities issued by employer included in plan assets |  |  |  | 3.0 | 3.0 |  |  |
Fair value of employer equity securities included in plan assets |  |  |  | $ 71.3 | $ 47.1 |  |  |
Dividends on employer stock received by plan |  |  |  |  | 0.7 |  |  |
Pension Benefits [Member] | U.S. [Member] | Global debt securities and cash [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Unfunded commitments |  |  |  | 84.0 |  |  |  |
Pension Benefits [Member] | U.S. [Member] | Private equity [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Unfunded commitments |  |  |  | 72.0 |  |  |  |
Pension Benefits [Member] | U.S. [Member] | Hedge funds [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Unfunded commitments |  |  |  | 11.0 |  |  |  |
Pension Benefits [Member] | U.S. [Member] | Real estate and other [Member] |  |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |
Unfunded commitments |  |  |  | 10.0 |  |  |  |
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 |  |  |  |  | $ 47.0 |  |  |

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