Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment revenue:
Revenue	$ 1,336.8	$ 1,203.5
Equity in earnings (losses)	13.0	11.3
Total segment revenue	1,349.8	1,214.8
Operating expenses:
Depreciation and amortization	31.2	24.9
Total operating expenses	1,307.4	1,150.8
Operating income	29.4	52.7
Segment Reconciling Items:
Revenue	1,336.8	1,203.5
Equity in earnings (losses)	13.0	11.3
Total segment revenue	1,349.8	1,214.8
Total segment operating expenses before restructuring charges	1,299.8	1,150.0
Segment Reporting Information Operating Income Before Restructuring Charges	37.0	53.5
Restructuring and acquisition charges	7.6	0.8
Operating income	29.4	52.7
Reportable Subsegments [Member] | Americas [Member]
Segment revenue:
Revenue	603.5	554.2
Equity in earnings (losses)	0.3	0.3
Total segment revenue	603.8	554.5
Gross contract costs	(48.8)	(52.9)
Total segment fee revenue	555.0	501.6
Operating expenses:
Compensation, operating and administrative expenses	554.6	503.5
Depreciation and amortization	18.9	15.6
Total operating expenses	573.5	519.1
Gross contract costs	(48.8)	(52.9)
Total fee based segment operating expenses	524.7	466.2
Operating income	30.3	35.4
Segment Reconciling Items:
Revenue	603.5	554.2
Equity in earnings (losses)	0.3	0.3
Total segment revenue	603.8	554.5
Operating income	30.3	35.4
Reportable Subsegments [Member] | EMEA [Member]
Segment revenue:
Revenue	369.4	325.8
Equity in earnings (losses)	(0.1)	(0.4)
Total segment revenue	369.3	325.4
Gross contract costs	(112.9)	(71.9)
Total segment fee revenue	256.4	253.5
Operating expenses:
Compensation, operating and administrative expenses	373.7	323.1
Depreciation and amortization	7.6	5.2
Total operating expenses	381.3	328.3
Gross contract costs	(112.9)	(71.9)
Total fee based segment operating expenses	268.4	256.4
Operating income	(12.0)	(2.9)
Segment Reconciling Items:
Revenue	369.4	325.8
Equity in earnings (losses)	(0.1)	(0.4)
Total segment revenue	369.3	325.4
Operating income	(12.0)	(2.9)
Reportable Subsegments [Member] | Asia Pacific [Member]
Segment revenue:
Revenue	263.4	237.8
Equity in earnings (losses)	0.1	(0.1)
Total segment revenue	263.5	237.7
Gross contract costs	(57.6)	(49.6)
Total segment fee revenue	205.9	188.1
Operating expenses:
Compensation, operating and administrative expenses	261.5	229.7
Depreciation and amortization	4.1	3.6
Total operating expenses	265.6	233.3
Gross contract costs	(57.6)	(49.6)
Total fee based segment operating expenses	208.0	183.7
Operating income	(2.1)	4.4
Segment Reconciling Items:
Revenue	263.4	237.8
Equity in earnings (losses)	0.1	(0.1)
Total segment revenue	263.5	237.7
Operating income	(2.1)	4.4
Investment Management [Member]
Segment revenue:
Revenue	100.5	85.7
Equity in earnings (losses)	12.7	11.5
Total segment revenue	113.2	97.2
Operating expenses:
Compensation, operating and administrative expenses	78.8	68.8
Depreciation and amortization	0.6	0.5
Total operating expenses	79.4	69.3
Operating income	33.8	27.9
Segment Reconciling Items:
Revenue	100.5	85.7
Equity in earnings (losses)	12.7	11.5
Total segment revenue	113.2	97.2
Operating income	$ 33.8	$ 27.9