Schedule II - Valuation and Qualifying Accounts and Reserves (Detail) - Allowance for Uncollectible Accounts - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 1.5	$ 1.4	$ 2.0
Charged to Income		2.7	2.7	2.7
Other Charges		0.0	0.0	0.0
Payments & Deductions	[1]	3.0	2.6	3.3
Balance at End of Period		$ 1.2	$ 1.5	$ 1.4

[1] Write-off of individual bad debt accounts