CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 67,020	$ 14,005	$ 83,506	$ 27,876
Other comprehensive income (loss), net of tax:
Cumulative foreign currency translation adjustments	5,154	5,347	9,287	(4,756)
Unrealized gains (losses) on marketable securities, net of tax effects of $0 during each of the three and six months ended March 31, 2018, and $0 during each of the three and six months ended March 31, 2017		9		(2)
Actuarial gains (losses), net of tax effects of $0 and ($2) during the three and six months ended March 31, 2018, $3 and $5 during the three and six months ended March 31, 2017	6	(16)	(3)	(6)
Total other comprehensive income (loss), net of tax	5,160	5,340	9,284	(4,764)
Comprehensive income	$ 72,180	$ 19,345	$ 92,790	$ 23,112