Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($)
Goodwill [Roll Forward]
Beginning balance	$ 2,675,402
Goodwill of acquired businesses	429
Ending balance	2,675,831
0
Goodwill [Roll Forward]
Beginning balance	1,354,667
Goodwill of acquired businesses	(133)
Ending balance	1,354,534
National Programs
Goodwill [Roll Forward]
Beginning balance	901,294
Goodwill of acquired businesses	0
Ending balance	901,294
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	284,869
Goodwill of acquired businesses	(127)
Ending balance	284,742
Services
Goodwill [Roll Forward]
Beginning balance	134,572
Goodwill of acquired businesses	689
Ending balance	$ 135,261