Revenue Recognition - Balance Sheet Impact of Adoption of ASC 606 (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	$ 629,597	$ 694,221
ASSETS
Contract assets	157,918	142,882
Deferred tax assets	30,581	8,607
LIABILITIES AND EQUITY
Contract liabilities	154,609	143,270
Income taxes payable	1,877	8,272
Equity:
Retained earnings	1,026,836	$ 944,965
Recognition Under Previous Guidance
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	634,972
ASSETS
Contract assets	165,530
Deferred tax assets	29,461
LIABILITIES AND EQUITY
Contract liabilities	164,868
Income taxes payable	1,597
Equity:
Retained earnings	1,028,724
Impact of the Adoption of ASC 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts Receivable, Net, Current	(5,375)
ASSETS
Contract assets	(7,612)
Deferred tax assets	1,120
LIABILITIES AND EQUITY
Contract liabilities	(10,259)
Income taxes payable	280
Equity:
Retained earnings	$ (1,888)