Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Net Income Per Share [Abstract]
Net income	$ 77,663	$ 69,872	$ 61,379	$ 98,527	$ 94,837	$ 85,905	$ 116,087	$ 130,905	$ 307,441	$ 427,734	$ 453,689
Weighted average shares - basic (shares)	61,149	60,978	60,869	60,649	60,283	59,981	59,686	60,216	60,911	60,044	61,262
Weighted Average Number Diluted Shares Outstanding Adjustment									467	412	751
Weighted Average Number of Shares Outstanding, Diluted	61,636	61,415	61,084	60,815	60,448	60,196	60,029	60,645	61,378	60,456	62,013
Net income per share - basic (USD per share)	$ 1.27	$ 1.15	$ 1.01	$ 1.62	$ 1.57	$ 1.43	$ 1.94	$ 2.17	$ 5.05	$ 7.12	$ 7.41
Net income per share - diluted (USD per share)	$ 1.26	$ 1.14	$ 1.00	$ 1.62	$ 1.57	$ 1.43	$ 1.93	$ 2.16	$ 5.01	$ 7.08	$ 7.32