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

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.

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

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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

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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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

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