Basis of Presentation and Summary of Significant Accounting Policies - Transactions Affecting Allowance for Doubtful Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Balance, beginning of year	$ 35.9	$ 26.0	$ 27.0
Provisions charged to expense	3.2	12.1	17.8
Write-offs and other	(6.7)	(2.2)	(18.8)
Balance, end of year	$ 32.4	$ 35.9	$ 26.0