Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Goodwill [Roll Forward]
Beginning balance	$ 2,716,079
Goodwill of acquired businesses	31,367
Ending balance	2,747,446
Services
Goodwill [Roll Forward]
Beginning balance	1,386,248
Goodwill of acquired businesses	31,356
Ending balance	1,417,604
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	11
Ending balance	286,109
Services
Goodwill [Roll Forward]
Beginning balance	135,261
Goodwill of acquired businesses	0
Ending balance	$ 135,261