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Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Revenue Recognition (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019	Oct. 01, 2018
Revenue Recognition
Retained earnings	$ 451,364		$ 451,364		$ 599,548
Revenue	$ 3,245,737	$ 3,412,605	6,481,347	$ 6,768,943
Subcontractor and other direct costs			$ 3,400,000	$ 3,700,000
Accounting Standards Update 2014-09
Revenue Recognition
Retained earnings						$ (7,000)