Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Goodwill [Roll Forward]
Beginning balance	$ 2,716,079
Goodwill of acquired businesses	129,775
Ending balance	2,845,854
Services
Goodwill [Roll Forward]
Beginning balance	1,386,248
Goodwill of acquired businesses	124,230
Ending balance	1,510,478
National Programs
Goodwill [Roll Forward]
Beginning balance	908,472
Goodwill of acquired businesses	0
Ending balance	908,472
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	286,098
Goodwill of acquired businesses	5,545
Ending balance	291,643
Services
Goodwill [Roll Forward]
Beginning balance	135,261
Goodwill of acquired businesses	0
Ending balance	$ 135,261