Restructuring Costs Restructuring Costs Recognized (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	$ 1.9	$ 46.2	$ 63.2
General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	1.9	43.4	54.3
Net gain (loss) from asset sales, inclusive of restructuring costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.0	1.4	8.6
Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.0	1.4	0.3
Termination benefits
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	1.0	12.3	32.3
Termination benefits | General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	1.0	12.2	25.7
Termination benefits | Net gain (loss) from asset sales, inclusive of restructuring costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.0	0.1	6.6
Termination benefits | Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.0	0.0	0.0
Office lease termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		0.6	1.0
Office lease termination costs | General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		0.6	1.0
Office lease termination costs | Net gain (loss) from asset sales, inclusive of restructuring costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		0.0	0.0
Office lease termination costs | Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		0.0	0.0
Accelerated Share Based Compensation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.5	12.6	11.0
Accelerated Share Based Compensation [Member] | General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.5	11.1	8.8
Accelerated Share Based Compensation [Member] | Net gain (loss) from asset sales, inclusive of restructuring costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.0	1.5	2.2
Accelerated Share Based Compensation [Member] | Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.0	0.0	0.0
Retention expense (including share-based compensation)
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.4	19.5	18.8
Retention expense (including share-based compensation) | General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.4	19.5	18.8
Retention expense (including share-based compensation) | Net gain (loss) from asset sales, inclusive of restructuring costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	0.0	0.0	0.0
Retention expense (including share-based compensation) | Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	$ 0.0	0.0	0.0
Pension and Medical Plan curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		1.2	0.1
Pension and Medical Plan curtailment | General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		0.0	0.0
Pension and Medical Plan curtailment | Net gain (loss) from asset sales, inclusive of restructuring costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		(0.2)	(0.2)
Pension and Medical Plan curtailment | Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		$ 1.4	$ 0.3