Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
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

Aggregate dividends paid during the period for each share of common stock outstanding.

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

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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.

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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

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The net result for the period of deducting operating expenses from operating revenues.

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The total amount of operating revenues recognized during the period.

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Quarterly Financial Information [Line Items]

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