Unrecognized tax benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Abstract]
Balance at Beginning of Year	$ 1.5	$ 0.8	$ 19.8
Additions to Tax Positions Taken During Prior Years	0.0	0.7	0.0
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	0.0	0.0	0.0
Settlements with Tax Authorities	0.0	0.0	(19.0)
Lapse of the Applicable Statute of Limitations	(0.1)	0.0	0.0
Balance at End of Year	1.4	1.5	0.8
Less Tax Attributable to Temporary Items Included Above	0.0	0.0	0.0
Total Unrecognized Tax Benefits that if Recognized Would Affect the Effective Tax Rate	$ 1.4	$ 1.5	$ 0.8