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Employee Benefits, Defined Benefit Plans (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2015 | Mar. 31, 2015 | Sep. 30, 2015 | Dec. 31, 2014 | Dec. 31, 2013
VEBA Postretirement Medical Obligations |  |  |  |  |
Accrued annual contribution to VEBAs | $ 0.0 |  | $ 0.0 | $ 13.7 |
VEBAs |  |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |  |
Cash flow in determining VEBA obligation |  |  | 20.0 |  |
Accrued annual contribution to VEBAs |  |  |  | 13.7 | $ 16.0
VEBAs | Maximum |  |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |  |
Administrative expenses of the VEBAs | 0.3 |  | 0.3 |  |
Variable cash contribution obligation to VEBAs | 20.0 |  | $ 20.0 |  |
VEBAs | Annual Cash Flows up to $20 Million |  |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |  |
Postretirement medical plan contribution obligation percentage |  |  | 10.00% |  |
VEBAs | Annual Cash Flows in Excess of $20 Million |  |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |  |
Postretirement medical plan contribution obligation percentage |  |  | 20.00% |  |
Union VEBA |  |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |  |
Percent allocation of total contribution between VEBAs |  |  | 85.50% |  |
Accrued Veba contingent contribution - total | 44.1 |  | $ 44.1 |  |
Defined Benefit Plan, Benefit Obligation |  |  |  | 391.5 |
Defined Benefit Plan, Fair Value of Plan Assets |  |  |  | $ 731.6 |
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments |  | $ 307.8 |  |  |
Income Tax Expense (Benefit) | $ 184.4 | $ 184.4 |  |  |
Salaried VEBA |  |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |  |
Percent allocation of total contribution between VEBAs |  |  | 14.50% |  |

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- Definition 1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date. + 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 (a) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Projected Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6522206 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Postretirement Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503904 + Details
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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=49178875&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920 + Details
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