Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) $ in Thousands, shares in Millions		3 Months Ended								12 Months Ended
	Nov. 01, 2017 Reporting_Unit	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Practice_Groups Reporting_Unit shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of practice groups | Practice_Groups										3
Fees earned under administrative service agreement										$ 156,400	$ 144,800	$ 137,500
Maximum maturity period of investments										3 months
Allowance for doubtful accounts		$ 13,827				$ 13,508				$ 13,827	13,508	12,659		$ 11,915
Software purchased or developed, estimated useful life										7 years
Goodwill		528,424				487,484				$ 528,424	487,484	447,685
Total assets		1,176,231				1,118,588				1,176,231	1,118,588
Total stockholders' equity		530,879				480,021				$ 530,879	480,021	427,948		399,845
Number of reporting units | Reporting_Unit	5									5
Intangible assets amortization period										7 years 1 month 6 days
Shares vesting term										4 years
Unsecured credit facility		400,000								$ 400,000
Contract payment terms										90 days
Total personnel costs										$ 596,400	544,800	502,800
Total occupancy costs										46,300	45,700	41,400
Income tax expense (benefit)		(3,368)	$ 6,172	$ 4,343	$ 16,141	(967)	$ 7,260	$ 5,306	$ 14,800	23,288	26,399	22,829
Retained Earnings [Member]
Summary Of Significant Accounting Policies [Line Items]
Total stockholders' equity		$ 345,302				$ 294,925				$ 345,302	$ 294,925	$ 254,860		$ 220,753
Accounting Standards Update 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase of diluted shares | shares										0.5
Income tax expense (benefit)										$ (3,800)
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible assets amortization period										2 years
Minimum [Member] | Accounting Standards Update 2014-09 | Retained Earnings [Member] | Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment to retained earnings													$ 1,500
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible assets amortization period										15 years
Maximum [Member] | Accounting Standards Update 2014-09 | Retained Earnings [Member] | Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment to retained earnings													$ 2,000