Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)			1 Months Ended	3 Months Ended
	Feb. 13, 2018	Oct. 30, 2017	Feb. 08, 2017	Mar. 31, 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]
Stock repurchase plan expiration date	Feb. 12, 2020	Oct. 30, 2019	Jan. 31, 2019
Stock repurchase plan period		24 months
Number of common stocks repurchased during the period				169,146
Stock repurchased, average costs per share				$ 99.74
Stock Repurchase Plan [Member] | Restricted Stock [Member]
Summary Of Significant Accounting Policy [Line Items]
Shares withheld to satisfy tax withholding obligations				108,909
Maximum [Member] | Stock Repurchase Plan [Member]
Summary Of Significant Accounting Policy [Line Items]
Stock repurchase plan, authorized amount	$ 100,000,000	$ 75,000,000	$ 50,000,000
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policy [Line Items]
Cumulative adjustment to stockholders’ equity					$ 103,400,000