Goodwill and Other Intangible Assets - Schedule of Changes in Goodwill by Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill Gross, Beginning	$ 4,323,246	$ 4,049,898
Accumulated impairment losses	(1,068,506)	(1,068,506)
Goodwill, Net of impairment losses	3,254,740	2,981,392
Purchase accounting entries related to acquisitions	471,204	143,822
Foreign exchange movement	(73,635)	129,526
Goodwill Gross, Ending	4,720,815	4,323,246
Accumulated impairment losses	(1,068,506)	(1,068,506)
Goodwill, Net of impairment losses	3,652,309	3,254,740
Americas [Member]
Goodwill [Line Items]
Goodwill Gross, Beginning	2,408,576	2,302,929
Accumulated impairment losses	(798,290)	(798,290)
Goodwill, Net of impairment losses	1,610,286	1,504,639
Purchase accounting entries related to acquisitions	450,380	104,654
Foreign exchange movement	(1,623)	993
Goodwill Gross, Ending	2,857,333	2,408,576
Accumulated impairment losses	(798,290)	(798,290)
Goodwill, Net of impairment losses	2,059,043	1,610,286
EMEA [Member]
Goodwill [Line Items]
Goodwill Gross, Beginning	1,156,458	1,047,295
Accumulated impairment losses	(138,631)	(138,631)
Goodwill, Net of impairment losses	1,017,827	908,664
Purchase accounting entries related to acquisitions	17,838	17,402
Foreign exchange movement	(51,753)	91,761
Goodwill Gross, Ending	1,122,543	1,156,458
Accumulated impairment losses	(138,631)	(138,631)
Goodwill, Net of impairment losses	983,912	1,017,827
Asia Pacific [Member]
Goodwill [Line Items]
Goodwill Gross, Beginning	166,108	150,706
Goodwill, Net of impairment losses	166,108	150,706
Purchase accounting entries related to acquisitions	8,096	4,198
Foreign exchange movement	(8,556)	11,204
Goodwill Gross, Ending	165,648	166,108
Goodwill, Net of impairment losses	165,648	166,108
Global Investment Management [Member]
Goodwill [Line Items]
Goodwill Gross, Beginning	505,441	462,305
Accumulated impairment losses	(44,922)	(44,922)
Goodwill, Net of impairment losses	460,519	417,383
Purchase accounting entries related to acquisitions	(5,110)	17,568
Foreign exchange movement	(11,703)	25,568
Goodwill Gross, Ending	488,628	505,441
Accumulated impairment losses	(44,922)	(44,922)
Goodwill, Net of impairment losses	443,706	460,519
Development Services [Member]
Goodwill [Line Items]
Goodwill Gross, Beginning	86,663	86,663
Accumulated impairment losses	(86,663)	(86,663)
Goodwill Gross, Ending	86,663	86,663
Accumulated impairment losses	$ (86,663)	$ (86,663)