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Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total Operating Revenues | $ 104.3 | $ 78.8 | $ 74.5 | $ 125.8 | $ 102.4 | $ 74.7 | $ 77.5 | $ 172.2 | $ 383.4 | $ 426.8 | $ 425.8
Operating Income | 21.3 | 11.1 | 9.5 | 23.4 | 18.5 | 7.3 | 8.9 | 28.4 | 65.3 | 63.1 | 60.0
Net Income (Loss) Applicable to Common | $ 10.2 | $ 3.5 | $ 2.5 | $ 10.9 | $ 9.3 | $ 1.7 | $ 1.7 | $ 13.6 | $ 27.1 | $ 26.3 | $ 24.7
Per Share Data: |  |  |  |  |  |  |  |  |  |  |
Earnings Per Common Share | $ 0.73 | $ 0.25 | $ 0.18 | $ 0.78 | $ 0.67 | $ 0.12 | $ 0.12 | $ 0.98 | $ 1.94 | $ 1.89 | $ 1.79
Dividends Paid Per Common Share | $ 0.355 | $ 0.355 | $ 0.355 | $ 0.355 | $ 0.350 | $ 0.350 | $ 0.350 | $ 0.350 | $ 1.42 | $ 1.40 | $ 1.38

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- Definition Aggregate dividends paid during the period for each share of common stock outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. + References No definition available. + Details
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- Definition Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=64863997&loc=d3e1377-109256 + Details
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