Recently Issued Accounting Pronouncements - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Recently Issued Accounting Pronouncements [Line Items]
Operating Income	$ 216.4	$ 212.7	$ 434.0	$ 417.9
Other non-operating items, net	$ (5.3)	$ 2.3	(14.8)	$ 1.1
Minimum | Accounting Standard Update 2017-07, Statement of Net Income Classification of Operating Income | Restatement Adjustment
Recently Issued Accounting Pronouncements [Line Items]
Operating Income			(5.0)
Other non-operating items, net			5.0
Maximum | Accounting Standard Update 2017-07, Statement of Net Income Classification of Operating Income | Restatement Adjustment
Recently Issued Accounting Pronouncements [Line Items]
Operating Income			(7.0)
Other non-operating items, net			$ 7.0