Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) $ in Thousands		12 Months Ended
	Nov. 01, 2018 Reporting_Unit	Dec. 31, 2018 USD ($) Practice_Groups Reporting_Unit	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of practice groups | Practice_Groups		3
Revenue		$ 922,003	$ 855,340	$ 799,832
Shares vesting term		4 years
Maximum maturity period of investments		3 months
Allowance for doubtful accounts		$ 13,389	13,827	13,508	$ 12,659
Software purchased or developed, estimated useful life		7 years
Goodwill		$ 564,300	528,424	487,484
Total assets		1,183,031	1,176,231
Total stockholders' equity		$ 593,663	530,879	480,021	$ 427,948
Intangible assets amortization period		6 years 9 months 18 days
Number of reporting units | Reporting_Unit	5	5
Net changes in the fair value of contingent consideration		$ (2,617)	1,494	994
Unsecured credit facility		400,000
Other Expense
Summary Of Significant Accounting Policies [Line Items]
Net changes in the fair value of contingent consideration		$ (2,600)
Other Income
Summary Of Significant Accounting Policies [Line Items]
Net changes in the fair value of contingent consideration			1,500	1,000
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible assets amortization period		2 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible assets amortization period		15 years
Financial Service [Member]
Summary Of Significant Accounting Policies [Line Items]
Revenue		$ 154,000	$ 156,400	$ 144,800