Restructuring Costs Costs recognized and remaining costs expected to be incurred (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	$ 111.3
Restructuring Costs, Expected Costs	0.0
Termination benefits
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	45.6
Restructuring Costs, Expected Costs	0.0
Office lease termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	1.6
Restructuring Costs, Expected Costs	0.0
Accelerated share-based compensation
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	24.1
Restructuring Costs, Expected Costs	0.0
Retention expense (including share-based compensation)
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	38.7
Restructuring Costs, Expected Costs	0.0
Pension and Medical Plan curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Cost Incurred to Date	1.3
Restructuring Costs, Expected Costs	$ 0.0