Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($)
Goodwill [Roll Forward]
Beginning balance	$ 2,675,402
Goodwill of acquired businesses	16,702
Ending balance	2,692,104
Services
Goodwill [Roll Forward]
Beginning balance	1,354,667
Goodwill of acquired businesses	7,929
Ending balance	1,362,596
National Programs
Goodwill [Roll Forward]
Beginning balance	901,294
Goodwill of acquired businesses	7,314
Ending balance	908,608
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	284,869
Goodwill of acquired businesses	770
Ending balance	285,639
Services
Goodwill [Roll Forward]
Beginning balance	134,572
Goodwill of acquired businesses	689
Ending balance	$ 135,261