Acquisitions - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 01, 2012
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration			$ 2,200,000
Goodwill	435,231,000	384,697,000	375,122,000
Consideration paid in cash	4,600,000	10,100,000	25,600,000
Number of common stock issued	67,000	184,000	402,000
Fair value of contingent consideration	3,900,000	1,100,000	1,100,000
Common stock issuable			41,314
Centric Insurance Agency (Centric), Clearview National Partners, LLC (Clearview), Tegrit Group (Tegrit), Sattler Insurance Agency (Sattler) and Lewis Birch & Richardo, LLC (LBR) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent arrangements arising from acquisitions	20,900,000
Contingent consideration	19,400,000
Goodwill	51,873,000
Centric Insurance Agency (Centric), Clearview National Partners, LLC (Clearview), Tegrit Group (Tegrit), Sattler Insurance Agency (Sattler) and Lewis Birch & Richardo, LLC (LBR) [Member] | Contingent Purchase Price Liability Current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	5,000,000
Centric Insurance Agency (Centric), Clearview National Partners, LLC (Clearview), Tegrit Group (Tegrit), Sattler Insurance Agency (Sattler) and Lewis Birch & Richardo, LLC (LBR) [Member] | Contingent Purchase Price Liability Non-current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	14,400,000
Associated Insurance Agents (AIA), Minneapolis, Minnesota and Knight Field Fabry LLP (Knight) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent arrangements arising from acquisitions		6,100,000
Contingent consideration		5,500,000
Goodwill		9,278,000
Consideration paid in cash		4,900,000
Number of businesses acquired		2
Guaranteed future consideration		400,000
CBIZ contingent consideration		5,500,000
Goodwill deductible for tax purposes		1,400,000
Associated Insurance Agents (AIA), Minneapolis, Minnesota and Knight Field Fabry LLP (Knight) [Member] | Contingent Purchase Price Liability Current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration		1,200,000
Associated Insurance Agents (AIA), Minneapolis, Minnesota and Knight Field Fabry LLP (Knight) [Member] | Contingent Purchase Price Liability Non-current [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration		4,300,000
Meridian Insurance Group, LLC (Meridian), Primarily Care, Inc. (PCI) Stoltz and Company, LTD., L.L.P (Stoltz), Trinity Risk Advisors, Inc.(Trinity), Strategic Employee Benefit Services-The Pruett Group, Inc. (SEBS-Pruett), ProMedical, Inc. (ProMedical), the employee benefit division of Leavitt Pacific Insurance Brokers, Inc. (Leavitt), Diversified Industries, Inc. d/b/a Payroll Control Systems (PCS) and PHBV Partners, L.L.P. (PHBV) [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent arrangements arising from acquisitions			18,500,000
Contingent consideration			15,400,000
Goodwill			57,190,000
Consideration paid in cash			74,200,000
Number of businesses acquired			8
Guaranteed future consideration			1,700,000
Goodwill deductible for tax purposes			53,000,000
Consideration paid in common stock			3,600,000
Short-term notes payable			4,500,000
Meridian Insurance Group, LLC (Meridian), Primarily Care, Inc. (PCI) Stoltz and Company, LTD., L.L.P (Stoltz), Trinity Risk Advisors, Inc.(Trinity), Strategic Employee Benefit Services-The Pruett Group, Inc. (SEBS-Pruett), ProMedical, Inc. (ProMedical), the employee benefit division of Leavitt Pacific Insurance Brokers, Inc. (Leavitt), Diversified Industries, Inc. d/b/a Payroll Control Systems (PCS) and PHBV Partners, L.L.P. (PHBV) [Member] | Other Current Liabilities [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration			4,600,000
Meridian Insurance Group, LLC (Meridian), Primarily Care, Inc. (PCI) Stoltz and Company, LTD., L.L.P (Stoltz), Trinity Risk Advisors, Inc.(Trinity), Strategic Employee Benefit Services-The Pruett Group, Inc. (SEBS-Pruett), ProMedical, Inc. (ProMedical), the employee benefit division of Leavitt Pacific Insurance Brokers, Inc. (Leavitt), Diversified Industries, Inc. d/b/a Payroll Control Systems (PCS) and PHBV Partners, L.L.P. (PHBV) [Member] | Other Non-current Liabilities [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration			10,800,000
Acquisition of Client Lists [Member]
Business Acquisition, Contingent Consideration [Line Items]
Contingent consideration	200,000	300,000
Number of client list purchased	4	3
Consideration paid in cash	$ 1,000,000	$ 300,000		$ 2,500,000
Employee Services [Member] | Acquisition of Client Lists [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased	1	2
Financial Services Practice Group [Member] | Acquisition of Client Lists [Member]
Business Acquisition, Contingent Consideration [Line Items]
Number of client list purchased	3	1