Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule Of Pre Tax Income [Line Items]
Excess tax benefits on share based payments	$ 1.7	$ 3.8	$ 3.7	$ 14.8
U.S. federal tax at statutory rate			21.00%		35.00%
Provisional tax charge recorded due to enactment of Tax Act			$ 11.7
Re-measurement of U.S. deferred tax assets			6.7
Repatriation taxes on accumulated undistributed foreign earnings			$ 5.0
Reduction in provisional tax charge	$ 0.4
New York State [Member] | Earliest Tax Year [Member]
Schedule Of Pre Tax Income [Line Items]
Income tax year under examination			2010
New York State [Member] | Latest Tax Year [Member]
Schedule Of Pre Tax Income [Line Items]
Income tax year under examination			2015