Restructuring (Rollforward of Restructuring Liability) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2020
Restructuring Cost and Reserve [Line Items]
Total charges	$ 84.0	$ 10.1	$ 119.0
Restructuring Reserve [Roll Forward]
Balance as of beginning of period	5.3	0.0
Non-cash charges			24.9
Accrued charges	84.0	10.1	119.0
Payments	(44.8)	(4.8)
Balance as of end of period	44.5	5.3	44.5
Minimum
Restructuring Cost and Reserve [Line Items]
Estimated restructuring expenses remaining	150.0		150.0
Maximum
Restructuring Cost and Reserve [Line Items]
Estimated restructuring expenses remaining	175.0		175.0
Employee Compensation
Restructuring Cost and Reserve [Line Items]
Total charges	$ 80.1	4.9	$ 104.5
Estimated restructuring expenses remaining (percent)	60.00%		60.00%
Restructuring Reserve [Roll Forward]
Balance as of beginning of period	$ 3.5	0.0
Non-cash charges			$ 19.5
Accrued charges	80.1	4.9	104.5
Payments	(39.1)	(1.4)
Balance as of end of period	44.5	3.5	44.5
Other Expenses
Restructuring Cost and Reserve [Line Items]
Total charges	3.9	5.2	14.5
Restructuring Reserve [Roll Forward]
Balance as of beginning of period	1.8	0.0
Non-cash charges			5.4
Accrued charges	3.9	5.2	14.5
Payments	(5.7)	(3.4)
Balance as of end of period	$ 0.0	$ 1.8	$ 0.0