Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Net Income Per Share [Abstract]
Net income	$ 94,837	$ 85,905	$ 116,087	$ 130,905	$ 132,881	$ 122,743	$ 109,636	$ 88,429	$ 427,734	$ 453,689	$ 420,761
Weighted average shares - basic (shares)	60,283	59,981	59,686	60,216	60,462	60,970	61,420	62,195	60,044	61,262	64,173
Weighted Average Number Diluted Shares Outstanding Adjustment									412	751	602
Weighted Average Number of Shares Outstanding, Diluted	60,448	60,196	60,029	60,645	61,070	61,633	62,059	62,550	60,456	62,013	64,775
Net income per share - basic (USD per share)	$ 1.57	$ 1.43	$ 1.94	$ 2.17	$ 2.20	$ 2.01	$ 1.79	$ 1.42	$ 7.12	$ 7.41	$ 6.56
Net income per share - diluted (USD per share)	$ 1.57	$ 1.43	$ 1.93	$ 2.16	$ 2.18	$ 1.99	$ 1.77	$ 1.41	$ 7.08	$ 7.32	$ 6.50