Fair Value Measurements (Assets and Liabilities Measured on a Recurring Basis) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in real estate ventures accounted for at fair value	$ 202.3	$ 155.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency forward contracts receivable	18.5	9.5
Deferred compensation plan assets	141.5	134.3
Total assets at fair value	160.0	143.8
Foreign currency forward contracts payable	3.9	21.2
Deferred compensation plan liabilities	138.5	129.4
Business Combination, Contingent Consideration, Liability	0.0	0.0
Total liabilities at fair value	142.4	150.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency forward contracts receivable	0.0	0.0
Deferred compensation plan assets	0.0	0.0
Total assets at fair value	0.0	0.0
Foreign currency forward contracts payable	0.0	0.0
Deferred compensation plan liabilities	0.0	0.0
Business Combination, Contingent Consideration, Liability	158.8	127.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0.7
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	32.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(1.7)
Total liabilities at fair value	$ 158.8	$ 127.3