New Accounting Pronouncements - Narrative (Detail) - Accounting Standards Update 2017-07 - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
New accounting pronouncement effect on operating results	$ (4.2)	$ (12.5)
Other Non-operating Income [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Effect on other non-operating income from the adoption of a new accounting pronouncement	$ 4.2	$ 12.5