Restructuring Activities - Transformation and workforce optimization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 0.0
Efficiency And Cost Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 168.0	$ 126.0
Efficiency And Cost Reduction | Corporate, Non-Segment
Restructuring Cost and Reserve [Line Items]
Restructuring charges	14.0	32.0
Efficiency And Cost Reduction | Advisory Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	78.0	51.0
Efficiency And Cost Reduction | Global Workplace Solutions | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	52.0	31.0
Efficiency And Cost Reduction | Real Estate Investments | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	24.0	12.0
Employee separation benefits
Restructuring Cost and Reserve [Line Items]
Restructuring charges	82.0	82.0
Employee separation benefits | Efficiency And Cost Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring charges	82.0	81.0
Employee separation benefits | Efficiency And Cost Reduction | Corporate, Non-Segment
Restructuring Cost and Reserve [Line Items]
Restructuring charges	11.0	19.0
Employee separation benefits | Efficiency And Cost Reduction | Advisory Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	26.0	33.0
Employee separation benefits | Efficiency And Cost Reduction | Global Workplace Solutions | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	32.0	20.0
Employee separation benefits | Efficiency And Cost Reduction | Real Estate Investments | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	13.0	9.0
Lease exit costs | Efficiency And Cost Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring charges	45.0	13.0
Lease exit costs | Efficiency And Cost Reduction | Corporate, Non-Segment
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.0	0.0
Lease exit costs | Efficiency And Cost Reduction | Advisory Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	39.0	10.0
Lease exit costs | Efficiency And Cost Reduction | Global Workplace Solutions | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.0	3.0
Lease exit costs | Efficiency And Cost Reduction | Real Estate Investments | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	4.0	0.0
Professional fees
Restructuring Cost and Reserve [Line Items]
Restructuring charges	32.0	23.0
Professional fees | Efficiency And Cost Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring charges	32.0	24.0
Professional fees | Efficiency And Cost Reduction | Corporate, Non-Segment
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2.0	13.0
Professional fees | Efficiency And Cost Reduction | Advisory Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	7.0	3.0
Professional fees | Efficiency And Cost Reduction | Global Workplace Solutions | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	19.0	5.0
Professional fees | Efficiency And Cost Reduction | Real Estate Investments | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	4.0	3.0
Subtotal | Efficiency And Cost Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring charges	159.0	118.0
Subtotal | Efficiency And Cost Reduction | Corporate, Non-Segment
Restructuring Cost and Reserve [Line Items]
Restructuring charges	14.0	32.0
Subtotal | Efficiency And Cost Reduction | Advisory Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	72.0	46.0
Subtotal | Efficiency And Cost Reduction | Global Workplace Solutions | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	52.0	28.0
Subtotal | Efficiency And Cost Reduction | Real Estate Investments | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	21.0	12.0
Depreciation expense | Efficiency And Cost Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring charges	9.0	8.0
Depreciation expense | Efficiency And Cost Reduction | Corporate, Non-Segment
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.0	0.0
Depreciation expense | Efficiency And Cost Reduction | Advisory Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	6.0	5.0
Depreciation expense | Efficiency And Cost Reduction | Global Workplace Solutions | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.0	3.0
Depreciation expense | Efficiency And Cost Reduction | Real Estate Investments | Operating Segments
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 3.0	$ 0.0