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Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 116.4	$ 85.3	$ 84.4	$ 152.1	$ 125.6	$ 88.2	$ 84.5	$ 145.8	$ 438.2	$ 444.1	$ 406.2
Operating Income	22.0	10.0	12.3	28.8	22.2	10.3	10.6	28.1	73.1	71.2	75.4
Net Income Applicable to Common	$ 11.4	$ 2.3	$ 4.0	$ 26.5	$ 11.0	$ 2.8	$ 3.6	$ 15.6	$ 44.2	$ 33.0	$ 29.0
Per Share Data:
Earnings Per Common Share	$ 0.77	$ 0.15	$ 0.27	$ 1.78	$ 0.74	$ 0.19	$ 0.24	$ 1.06	$ 2.97	$ 2.23	$ 2.06
Dividends Paid Per Common Share	$ 0.37	$ 0.37	$ 0.37	$ 0.37	$ 0.365	$ 0.365	$ 0.365	$ 0.365	$ 1.48	$ 1.46	$ 1.44

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- Definition

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

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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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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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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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