Derivatives and Fair Value Measurements - Long-term debt (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net carrying amount	$ 1,600.1	$ 1,310.8
Carrying amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total debt at par value	1,646.9	1,367.2
Less: Unamortized debt issuance costs and original issue discount	(46.8)	(56.4)
Net carrying amount	1,600.1	1,310.8
Estimated fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value	$ 886.9	$ 1,271.1

Amount of short-term and long-term debt and lease obligation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

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Amount of long-term debt and lease obligation, including portion classified as current.

+ References

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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