Asset Retirement Obligations (Narrative) (Details) $ in Millions	Dec. 31, 2017 USD ($)
asset retirement obligations [Line Items]
Surety bonds outstanding	$ 31.2
Letters of credit outstanding	7.4
Cash collateral for surety bond	2.6
Asset retirement obligations
asset retirement obligations [Line Items]
Surety bonds outstanding	$ 531.7

Carrying value of amounts transferred to third parties for security purposes that are expected to be returned or applied towards payment after one year or beyond the operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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