Restructuring and Other Charges, net (Accrual Activity Relating to Restructuring and Other Charges) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Restructuring Cost and Reserve [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income			$ 11,674
Restructuring [Roll Forward]
Balance at September 30, 2019			7,209
Restructuring charges	$ 2,820	$ 6,978	6,868	$ 13,000
Restructuring Reserve, Accrual Adjustment			1,263
Cash payments			(9,320)
Balance at March 31, 2020	17,694		17,694
Personnel
Restructuring Cost and Reserve [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income			0
Restructuring [Roll Forward]
Balance at September 30, 2019			3,587
Restructuring charges	1,422	6,352	4,335	12,565
Restructuring Reserve, Accrual Adjustment			0
Cash payments			(6,172)
Balance at March 31, 2020	1,750		1,750
Facilities
Restructuring Cost and Reserve [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income			11,674
Restructuring [Roll Forward]
Balance at September 30, 2019			3,622
Restructuring charges	1,398	$ 626	2,533	$ 476
Restructuring Reserve, Accrual Adjustment			1,263
Cash payments			(3,148)
Balance at March 31, 2020	$ 15,944		$ 15,944