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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Computation Of Basic And Diluted Earnings Per Common Share [Line Items]
Earnings Available to Common Shareholders									$ 29.0	$ 27.1	$ 26.3
Weighted Average Common Shares Outstanding-Basic									14,095	13,990	13,917
Plus: Diluted Effect of Incremental Shares									7	6	3
Weighted Average Common Shares Outstanding-Diluted									14,102	13,996	13,920
Earnings per Share-Basic and Diluted	$ 0.79	$ 0.16	$ 0.23	$ 0.88	$ 0.73	$ 0.25	$ 0.18	$ 0.78	$ 2.06	$ 1.94	$ 1.89