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Acquisitions (Fourth Quarter 2015) - Additional Information (Detail) - Benefit and Insurance Services Practice Group [Member]
 12 Months Ended
Dec. 31, 2017
Participant
Pension Resource Group Inc [Member]  
Business Acquisition, Contingent Consideration [Line Items]  
Acquired entity, name Pension Resource Group, Inc
Effective date of acquisition Oct. 01, 2015
Pension Resource Group Inc [Member] | Minimum [Member]  
Business Acquisition, Contingent Consideration [Line Items]  
Number of clients of acquired entity 500
Pension Resource Group Inc [Member] | Maximum [Member]  
Business Acquisition, Contingent Consideration [Line Items]  
Number of clients of acquired entity 60,000
Cottonwood [Member]  
Business Acquisition, Contingent Consideration [Line Items]  
Acquired entity, name Cottonwood Group, Inc
Effective date of acquisition Dec. 01, 2015