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Retirement Benefits - Narrative (Details) shares in Millions, $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jul. 31, 2016 USD ($) | Dec. 31, 2012 | Dec. 31, 2017 | Dec. 31, 2016 USD ($) retiree shares | Dec. 31, 2015 USD ($) shares | Dec. 31, 2014 USD ($) | Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Employer contributions | $ 115.0 |  |  |  |  |  |
Projected Retirement Benefit Expense In Next 12 months |  |  |  | $ 71.0 |  |  |
Defined contribution plan- Company contribution (percent) |  |  |  |  | 6.50% |  |
Defined contribution plan- Company matching contribution (percent) |  |  |  |  | 3.00% |  |
Other postretirement benefit costs for a defined contribution plan |  |  |  | 34.5 | $ 41.2 | $ 21.9 |
US Qualified Plan [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Estimated employer contributions in next 12 months |  |  |  | $ 135.0 |  |  |
Percentage of total assets |  |  |  | 96.00% |  |  |
Number of shares of equity securities issued by employer included in plan assets | shares |  |  |  | 3.0 | 3.0 |  |
Fair value of employer equity securities included in plan assets |  |  |  | $ 47.1 | $ 33.2 |  |
Dividends on employer stock received by plan |  |  |  | 0.7 | 1.8 |  |
Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Employer contributions |  |  |  | 125.1 | 10.3 |  |
Projected Retirement Benefit Expense In Next 12 months |  |  |  | $ 48.0 |  |  |
Number of retirees removed from plan through annual buy-out | retiree |  |  |  | 3,700 |  |  |
Benefits paid for small annuity buyout |  |  |  | $ 47.0 |  |  |
Accumulated benefit obligation |  |  |  | $ 2,710.7 | $ 2,767.0 |  |
Expected long-term rate of return on assets |  |  |  | 8.00% | 8.00% | 8.25% |
Actual return on plan assets |  | 8.00% |  | 5.30% | (1.20%) | 6.50% | 14.30%
Fair value of plan assets |  |  |  | $ 1,894.6 | $ 1,962.3 | $ 2,204.4 |
Other Pension Plans, Defined Benefit [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Estimated employer contributions in next 12 months |  |  |  | 10.0 |  |  |
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 |  |  |
Assumed increase in per capita cost of covered benefits in next 12 months |  |  |  | 7.20% |  |  |
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% | 8.30% |
Fair value of plan assets |  |  |  | $ 0.7 | $ 1.8 | $ 2.9 |
Global debt securities and cash [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Unfunded commitments |  |  |  | 124.0 |  |  |
Private equity [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Unfunded commitments |  |  |  | 102.0 |  |  |
Fair value of plan assets |  |  |  | 204.1 | 201.2 |  |
Real estate and other [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Unfunded commitments |  |  |  | 11.0 |  |  |
Fair value of plan assets |  |  |  | 96.5 | 186.5 |  |
Fair Value, Inputs, Level 3 [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  |  | 99.3 | 305.6 | 361.3 |
Fair Value, Inputs, Level 3 [Member] | Private equity [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  |  | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 3 [Member] | Private equity [Member] | Other Postretirement Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  |  | 0.7 |  |  |
Fair Value, Inputs, Level 3 [Member] | Real estate and other [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Fair value of plan assets |  |  |  | $ 0.0 | $ 0.0 |  |
Other Postretirement Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Other postretirement benefit costs for a defined contribution plan |  |  |  |  |  | $ 2.6 |
Forecast [Member] | Pension Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Expected long-term rate of return on assets |  |  | 7.75% |  |  |  |
Forecast [Member] | Other Postretirement Benefits [Member] |  |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |
Expected long-term rate of return on assets |  |  | 4.00% |  |  |  |
MultiemployerPlan1 [Member] | Pension Benefits [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% |  |

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- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Health Care Cost Trend Rate -URI http://asc.fasb.org/extlink&oid=6514820 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The ultimate trend rate for health care costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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