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, 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
Schedule Of Computation Of Basic And Diluted Earnings Per Common Share [Line Items]
Earnings Available to Common Shareholders									$ 27.1	$ 26.3	$ 24.7
Weighted Average Common Shares Outstanding - Basic									13,990	13,917	13,843
Plus: Diluted Effect of Incremental Shares									6	3	4
Weighted Average Common Shares Outstanding - Diluted									13,996	13,920	13,847
Earnings per Share-Basic and Diluted	$ 0.73	$ 0.25	$ 0.18	$ 0.78	$ 0.67	$ 0.12	$ 0.12	$ 0.98	$ 1.94	$ 1.89	$ 1.79