Basis of Presentation and Significant Accounting Policies - Additional Information (Details) $ in Thousands		3 Months Ended								12 Months Ended
	Nov. 01, 2020 reportingUnit	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) reportingUnit practiceGroup	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of practice groups | practiceGroup										3
Revenue		$ 211,110	$ 238,389	$ 236,943	$ 277,455	$ 203,138	$ 239,790	$ 235,498	$ 269,998	$ 963,897	$ 948,424	$ 922,003
Maximum maturity period of investments										3 months
Allowance for doubtful accounts		14,894				14,379				$ 14,894	14,379	13,389	$ 13,827
Goodwill		683,083				588,213				683,083	588,213	564,300
Assets		1,513,754				1,400,774				1,513,754	1,400,774
Total stockholders' equity		702,620				$ 659,238				$ 702,620	659,238	593,663	$ 530,879
Intangible assets amortization period										6 years 9 months 18 days
Number of reporting units | reportingUnit	5									5
Net changes in the fair value of contingent consideration										$ (629)	1,599	2,617
Unsecured credit facility		$ 400,000								400,000
Other income (expense), net
Summary Of Significant Accounting Policies [Line Items]
Net changes in the fair value of contingent consideration										$ 600	(1,600)	2,600
Performance Shares
Summary Of Significant Accounting Policies [Line Items]
Shares vesting term										3 years
Minimum
Summary Of Significant Accounting Policies [Line Items]
Shares vesting term										3 years
Intangible assets amortization period										2 years
Minimum | Capitalized software
Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful life										2 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Shares vesting term										4 years
Intangible assets amortization period										15 years
Maximum | Capitalized software
Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful life										7 years
Financial service
Summary Of Significant Accounting Policies [Line Items]
Revenue										$ 159,400	$ 157,600	$ 154,000