CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 3,421	$ 67,020	$ 17,836	$ 83,506
Other comprehensive income (loss), net of tax:
Cumulative foreign currency translation adjustments	(3,513)	5,154	(1,836)	9,287
Unrealized gains (losses) on marketable securities, net of tax effects of $0 during each of the three and six months ended March 31, 2019, and $0 during each of the three and six months ended March 31, 2018	232		111
Actuarial gains (losses), net of tax effects of $2 and $3 during the six months ended March 31, 2019 and 2018 respectively	(1)	6	(10)	(3)
Total other comprehensive income (loss), net of tax	(3,282)	5,160	(1,735)	9,284
Comprehensive income (loss)	$ 139	$ 72,180	$ 16,101	$ 92,790