Revenue Recognition (Backlog) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Jun. 30, 2020	Sep. 30, 2019
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining performance obligation	$ 3,700,000	$ 3,400,000
Trade receivables sold to a financial institution	0	31,100
Receipt of withheld proceeds from sale of trade receivables	5,521
U.S. federal government contracts
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Amounts billed	$ 53,300	$ 60,300
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining performance obligation (in percent)	30.00%
Contract term	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining performance obligation (in percent)	45.00%
Contract term	24 months