Notes Receivable from Employees - Additional information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) note	Dec. 31, 2016 USD ($) note	Dec. 31, 2015 USD ($)
Receivables [Abstract]
Notes outstanding | note	251	307
Amortization of forgivable loan expense | $	$ 26.8	$ 25.6	$ 26.0