ARO Trust (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Annual funding obligation	$ 36.4
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	116.0	$ 90.3
Available-for-sale securities, fair value	134.6	96.4
Cash And Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	12.6	5.0
Available-for-sale securities, fair value	12.6	5.0
U.S. Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	35.9	29.4
Available-for-sale securities, fair value	50.5	36.5
International Equity Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	20.7	19.2
Available-for-sale securities, fair value	24.6	18.6
Municipal Bond Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, amortized cost basis	46.8	36.7
Available-for-sale securities, fair value	$ 46.9	$ 36.3

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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+ References

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 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Minimum amount of other commitment maturing in the next fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

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