Reconciliation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Schedule Of Computation Of Basic And Diluted Earnings Per Common Share [Line Items]
Earnings Available to Common Shareholders									$ 32.2	$ 44.2	$ 33.0
Weighted Average Common Shares Outstanding—Basic									14,951	14,894	14,824
Plus: Diluted Effect of Incremental Shares									1	6	5
Weighted Average Common Shares Outstanding—Diluted									14,952	14,900	14,829
Earnings per Share—Basic and Diluted	$ 0.90	$ 0.02	$ 0.21	$ 1.02	$ 0.77	$ 0.15	$ 0.27	$ 1.78	$ 2.15	$ 2.97	$ 2.23