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Employee Benefits - Changes in the Fair Value of Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015
Pension Benefits |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | $ 2,755.1 | $ 1,444.6
Realized and unrealized gains | 199.4 | (62.1)
Ending balance at December 31 | 2,709.2 | 2,755.1
Pension Benefits | Level 3 |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 5.5 |
Ending balance at December 31 | 15.4 | 5.5
Pension Benefits | Private Placement |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 5.5 |
Ending balance at December 31 | 14.6 | 5.5
Pension Benefits | Private Placement | Level 3 |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 5.5 | 0.0
Realized and unrealized gains | 0.5 |
Purchases | 8.6 | 5.5
Ending balance at December 31 | 14.6 | 5.5
Pension Benefits | United States bonds |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 808.7 |
Ending balance at December 31 | 862.1 | 808.7
Pension Benefits | United States bonds | Level 3 |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.0 |
Realized and unrealized gains | 0.0 |
Purchases | 0.8 |
Ending balance at December 31 | 0.8 | 0.0
OPEB |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 749.8 | 333.5
Realized and unrealized gains | 51.5 | (15.6)
Ending balance at December 31 | 773.5 | 749.8
OPEB | Level 3 |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.4 |
Ending balance at December 31 | 1.3 | 0.4
OPEB | Private Placement |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.4 |
Ending balance at December 31 | 1.3 | 0.4
OPEB | Private Placement | Level 3 |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.4 | 0.0
Realized and unrealized gains | 0.1 |
Purchases | 0.8 | 0.4
Ending balance at December 31 | 1.3 | 0.4
OPEB | United States bonds |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 122.3 |
Ending balance at December 31 | 137.9 | 122.3
OPEB | United States bonds | Level 3 |  |
Reconciliation of changes in the fair value of pension plan assets |  |
Beginning balance at January 1 | 0.0 |
Ending balance at December 31 | $ 0.0 | $ 0.0

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- Definition The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost) -URI http://asc.fasb.org/extlink&oid=6504192 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)(1) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6504226 + Details
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- Definition The amount of increase in the plan assets attributed to a business combination. + 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 (b)(6) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + 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=64937038&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=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6749337 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6520858 + Details
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