Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Beginning balance	$ 2,586,683	$ 2,460,611
Goodwill of acquired businesses	93,376	136,381
Goodwill, Transfers	0
Goodwill disposed of relating to sales of businesses	(4,657)	(10,309)
Ending balance	2,675,402	2,586,683
Retail
Goodwill [Roll Forward]
Beginning balance	1,345,636	1,231,869
Goodwill of acquired businesses	13,117	113,767
Goodwill, Transfers	571
Goodwill disposed of relating to sales of businesses	(4,657)	0
Ending balance	1,354,667	1,345,636
National Programs
Goodwill [Roll Forward]
Beginning balance	901,866	886,095
Goodwill of acquired businesses	(1)	18,009
Goodwill, Transfers	(571)
Goodwill disposed of relating to sales of businesses	0	(2,238)
Ending balance	901,294	901,866
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	226,961	222,356
Goodwill of acquired businesses	57,908	4,605
Goodwill, Transfers	0
Goodwill disposed of relating to sales of businesses	0	0
Ending balance	284,869	226,961
Services
Goodwill [Roll Forward]
Beginning balance	112,220	120,291
Goodwill of acquired businesses	22,352	0
Goodwill, Transfers	0
Goodwill disposed of relating to sales of businesses	0	(8,071)
Ending balance	$ 134,572	$ 112,220