Restructuring Costs Restructuring Costs Recognized (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		$ 46.2		$ 63.2
General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		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		1.4		8.6
Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		1.4		0.3
Termination benefits
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		12.3		32.3
Termination benefits | General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		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		0.1		6.6
Termination benefits | Interest and other income (expense) [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		0.0		0.0
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		12.6		11.0
Accelerated Share Based Compensation [Member] | Total incurred to date [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		29.1
Accelerated Share Based Compensation [Member] | General and administrative expense [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs	[1]	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	[1]	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	[1]	0.0
Retention expense (including share-based compensation)
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Costs		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		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
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
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

[1] Accelerated share-based compensation represents the additional expense or loss recognized in the Consolidated Statements of Operations for the year ended December 31, 2019 and 2018. Total accelerated share-based compensation was $29.1 million and was determined based on the contractual vesting date, with $12.6 million and $11.0 million recognized in 2019 and 2018, respectively, as shown above, and the remaining amount recognized in prior periods.