Revenue Recognition (Nature of Goods and Services) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Change in Accounting Estimate [Line Items]
Operating income (loss)	$ 560.2	$ 539.2	$ 577.2	$ 561.0	$ 603.2	$ 569.7	$ 516.5	$ 455.0	$ 2,237.6	$ 2,144.4	$ 1,965.6
Contracts accounted for under percentage of completion
Change in Accounting Estimate [Line Items]
Operating income (loss)									$ 7.0	$ 37.0	$ 38.0