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Employee Benefits - Pension and Post Retirement Plan Assets (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | ---
International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets |  | $ 8.1 |
Pension Plan Assets |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 2,709.2 | 2,755.1 | $ 1,444.6
Fair value of plan assets subject to levelling | 1,343.0 | 1,181.0 |
Pension Plan Assets | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 61.7 | 46.6 |
Pension Plan Assets | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 274.0 | 136.0 |
Pension Plan Assets | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 54.7 | 103.9 |
Pension Plan Assets | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 862.1 | 808.7 |
Pension Plan Assets | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 75.9 | 80.3 |
Pension Plan Assets | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 14.6 | 5.5 |
Pension Plan Assets | Investments measured at net asset value |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 1,366.2 | 1,574.1 |
Pension Plan Assets | Level 1 |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 331.7 | 264.9 |
Pension Plan Assets | Level 1 | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 3.7 | 17.0 |
Pension Plan Assets | Level 1 | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 273.9 | 132.6 |
Pension Plan Assets | Level 1 | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 54.1 | 103.9 |
Pension Plan Assets | Level 1 | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 11.4 |
Pension Plan Assets | Level 1 | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
Pension Plan Assets | Level 1 | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
Pension Plan Assets | Level 2 |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 995.9 | 910.6 |
Pension Plan Assets | Level 2 | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 58.0 | 29.6 |
Pension Plan Assets | Level 2 | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.1 | 3.4 |
Pension Plan Assets | Level 2 | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.6 | 0.0 |
Pension Plan Assets | Level 2 | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 861.3 | 797.3 |
Pension Plan Assets | Level 2 | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 75.9 | 80.3 |
Pension Plan Assets | Level 2 | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
Pension Plan Assets | Level 3 |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 15.4 | 5.5 |
Pension Plan Assets | Level 3 | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
Pension Plan Assets | Level 3 | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
Pension Plan Assets | Level 3 | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
Pension Plan Assets | Level 3 | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.8 | 0.0 |
Pension Plan Assets | Level 3 | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
Pension Plan Assets | Level 3 | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 14.6 | 5.5 | 0.0
OPEB Plan Assets |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 773.5 | 749.8 | 333.5
Fair value of plan assets subject to levelling | 218.2 | 188.4 |
OPEB Plan Assets | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 32.2 | 11.5 |
OPEB Plan Assets | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 34.3 | 24.7 |
OPEB Plan Assets | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 3.7 | 21.4 |
OPEB Plan Assets | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 137.9 | 122.3 |
OPEB Plan Assets | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 8.8 |  |
OPEB Plan Assets | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 1.3 | 0.4 |
OPEB Plan Assets | Investments measured at net asset value |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 555.3 | 561.4 |
OPEB Plan Assets | Level 1 |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 66.6 | 56.8 |
OPEB Plan Assets | Level 1 | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 28.8 | 10.5 |
OPEB Plan Assets | Level 1 | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 34.3 | 24.6 |
OPEB Plan Assets | Level 1 | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 3.5 | 21.4 |
OPEB Plan Assets | Level 1 | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.3 |
OPEB Plan Assets | Level 1 | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 1 | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 2 |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 150.3 | 131.2 |
OPEB Plan Assets | Level 2 | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 3.4 | 1.0 |
OPEB Plan Assets | Level 2 | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.1 |
OPEB Plan Assets | Level 2 | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.2 | 0.0 |
OPEB Plan Assets | Level 2 | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 137.9 | 122.0 |
OPEB Plan Assets | Level 2 | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 8.8 | 8.1 |
OPEB Plan Assets | Level 2 | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 3 |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 1.3 | 0.4 |
OPEB Plan Assets | Level 3 | Cash and cash equivalents |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 3 | United states equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 3 | International equity |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 3 | United States bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 3 | International bonds |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 0.0 | 0.0 |
OPEB Plan Assets | Level 3 | Private Placement |  |  |
Employee Benefit Plans |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 1.3 | $ 0.4 | $ 0.0

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