Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Beginning balance	$ 2,460,611	$ 2,006,173
Goodwill of acquired businesses	136,381	513,951
Goodwill disposed of relating to sales of businesses	(10,309)	(59,513)
Ending balance	2,586,683	2,460,611
Retail
Goodwill [Roll Forward]
Beginning balance	1,231,869	1,141,485
Goodwill of acquired businesses	113,767	94,080
Goodwill disposed of relating to sales of businesses	0	(3,696)
Ending balance	1,345,636	1,231,869
National Programs
Goodwill [Roll Forward]
Beginning balance	886,095	475,596
Goodwill of acquired businesses	18,009	420,063
Goodwill disposed of relating to sales of businesses	(2,238)	(9,564)
Ending balance	901,866	886,095
Wholesale Brokerage
Goodwill [Roll Forward]
Beginning balance	222,356	268,562
Goodwill of acquired businesses	4,605	47
Goodwill disposed of relating to sales of businesses	0	(46,253)
Ending balance	226,961	222,356
Services
Goodwill [Roll Forward]
Beginning balance	120,291	120,530
Goodwill of acquired businesses	0	(239)
Goodwill disposed of relating to sales of businesses	(8,071)	0
Ending balance	$ 112,220	$ 120,291