Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information [Line Items]
Total Operating Revenues	$ 116.9	$ 87.4	$ 83.9	$ 130.4	$ 116.4	$ 85.3	$ 84.4	$ 152.1	$ 418.6	$ 438.2	$ 444.1
Operating Income	25.3	7.4	11.1	27.6	22.0	10.0	12.3	28.8	71.4	73.1	71.2
Net Income Applicable to Common	$ 13.6	$ 0.3	$ 3.1	$ 15.2	$ 11.4	$ 2.3	$ 4.0	$ 26.5	$ 32.2	$ 44.2	$ 33.0
Per Share Data:
Earnings Per Common Share	$ 0.90	$ 0.02	$ 0.21	$ 1.02	$ 0.77	$ 0.15	$ 0.27	$ 1.78	$ 2.15	$ 2.97	$ 2.23
Dividends Paid Per Common Share	$ 0.375	$ 0.375	$ 0.375	$ 0.375	$ 0.37	$ 0.37	$ 0.37	$ 0.37