Supplementary Information (Non-Cash Activity) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Asset retirement obligation incurred	$ 11.0	$ 18.0	$ 19.0
Asset retirement obligation change in estimated future cash outflows	88.0	(99.0)	115.0
Property, plant and equipment accruals	19.0	5.0	(346.0)
Capitalized long-term incentives	55.0	40.0	(10.0)
Property additions/dispositions	194.0	100.0	12.0
Common Shares Issued under Dividend Reinvestment Plan	$ 0.6	$ 0.9	$ 73.0

The increase (decrease) during the reporting period for capital accruals.

No definition available.

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- Definition

Amount of asset retirement obligations incurred during the period.

+ References

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Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 35
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No definition available.

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- Definition

Value of property contributed in noncash investing and financing activities.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
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Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(ii)
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- Definition

Value of stock issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the holder of the stock to reinvest dividends paid to them by the entity on new issues of stock by the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
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