Employee Benefit Plans (Fair Value Measurements Classified as Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Changes in Plan Assets:
Beginning fair value of plan assets	$ 169.8
Ending fair value of plan assets	164.9	$ 169.8
Fair Value, Inputs, Level 3 [Member]
Changes in Plan Assets:
Beginning fair value of plan assets	3.5
Ending fair value of plan assets	3.2	3.5
Coal Lease [Member]
Changes in Plan Assets:
Beginning fair value of plan assets	3.5
Ending fair value of plan assets	3.2	3.5
Coal Lease [Member] \| Fair Value, Inputs, Level 3 [Member]
Changes in Plan Assets:
Beginning fair value of plan assets	3.5	3.7
Change in fair value of plan assets	(0.3)	(0.2)
Ending fair value of plan assets	$ 3.2	$ 3.5

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.

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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant