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Nature of Operations and Summary of Significant Accounting Policies - Capitalized Interest (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Capitalized Interest |  |  |
Capitalized interest costs | $ 1.4 | $ 1.5 | $ 2.2

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- Definition Amount of interest capitalized, including amount of allowance for funds used during construction. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 835 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433 + Details
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- References No definition available. + Details
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