Acquisitions - Schedule of Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 448,611	$ 447,685
Model Consulting, Inc. (Model),Pension Resource Group, Inc. (PRG),Cottonwood Group, Inc. (Cottonwood)[Member]
Business Acquisition [Line Items]
Accounts receivable, net	325
Other assets	38
Identifiable intangible assets	1,005		$ 2,844
Accrued liabilities	(49)
Total identifiable net assets	1,319		2,844
Goodwill	988		6,865
Aggregate purchase price	$ 2,307		$ 9,709