Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Feb. 13, 2018	Jan. 01, 2016
Summary Of Significant Accounting Policy [Line Items]
Expected life of client relationships	10 years
Stock Repurchase Plan [Member]
Summary Of Significant Accounting Policy [Line Items]
Number of common stocks repurchased during the period	576,710
Stock repurchased, average costs per share	$ 103.82
Available authorized repurchase amount	$ 77,500,000
Stock repurchase plan expiration date	Feb. 12, 2020
Stock Repurchase Plan [Member] | Restricted Stock [Member]
Summary Of Significant Accounting Policy [Line Items]
Shares withheld to satisfy tax withholding obligations	166,473
Maximum [Member] | Stock Repurchase Plan [Member]
Summary Of Significant Accounting Policy [Line Items]
Stock repurchase plan, authorized amount		$ 100,000,000
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policy [Line Items]
Cumulative adjustment, increase in stockholders’ equity			$ 103,400,000