Restructuring Costs Restructuring Expense Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other Nonoperating Income (Expense) [Member]
Total restructuring costs	$ 0.1		$ (0.2)
Other Expense [Member]
Total restructuring costs		$ 0.0		$ 0.0
Termination Benefits [Member]
Costs recognized period from inception to June 30, 2019(1)	38.9		38.9
Total remaining costs expected to be incurred			0.0
Office lease Termination Costs [Member]
Costs recognized period from inception to June 30, 2019(1)	1.6		1.6
Total remaining costs expected to be incurred			0.0
Accelerated Share Based Compensation [Member]
Costs recognized period from inception to June 30, 2019(1)	21.0		21.0
Total remaining costs expected to be incurred			0.0
Retention Expense [Member]
Costs recognized period from inception to June 30, 2019(1)	29.7		29.7
Total remaining costs expected to be incurred			10.1
Pension curtailment [Member]
Costs recognized period from inception to June 30, 2019(1)	(0.2)		(0.2)
Total remaining costs expected to be incurred			0.0
Total [Member]
Termination benefits	(0.1)	3.6	6.7	7.0
Office lease termination costs	0.0	0.3	0.6	0.3
Accelerated share-based compensation(1)	1.3	1.2	9.7	4.0
Retention expense (including share-based compensation)	4.8	6.3	10.9	8.0
Pension and Medical Plan curtailment	0.1		(0.4)
Total restructuring costs	6.1	11.4	27.5	19.3
Costs recognized period from inception to June 30, 2019(1)	91.0		91.0
Total remaining costs expected to be incurred			10.1
General and Administrative Expense [Member]
Termination benefits	(0.1)	1.7	6.6	5.1
Office lease termination costs	0.0	0.3	0.6	0.3
Accelerated share-based compensation(1)	1.3	1.2	8.2	4.0
Retention expense (including share-based compensation)	4.8	6.3	10.9	8.0
Pension and Medical Plan curtailment	0.0		0.0
Total restructuring costs	6.0	9.5	26.3	17.4
Net gain (loss) from asset sales [Member]
Termination benefits	0.0	1.9	0.1	1.9
Office lease termination costs	0.0	0.0	0.0	0.0
Accelerated share-based compensation(1)	0.0	0.0	1.5	0.0
Retention expense (including share-based compensation)	0.0	0.0	0.0	0.0
Pension and Medical Plan curtailment	0.0		(0.2)
Total restructuring costs	0.0	1.9	1.4	1.9
Interest and other income (expense) [Member]
Termination benefits	0.0	0.0	0.0	0.0
Office lease termination costs	0.0	0.0	0.0	0.0
Accelerated share-based compensation(1)	0.0	0.0	0.0	0.0
Retention expense (including share-based compensation)	0.0	$ 0.0	0.0	$ 0.0
Pension and Medical Plan curtailment	0.1		(0.2)
Net gain (loss) from asset sales [Member] | Total [Member]
Pension and Medical Plan curtailment			(0.5)
Total incurred to date [Member]
Accelerated share-based compensation(1)	$ 2.8		$ 21.7