Earnings 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
Basic Earnings Per Share
Net Income	$ 65,333	$ 117,791	$ 239,646	$ 178,673	$ 58,406	$ 80,466	$ 214,888	$ 157,646	$ 601,443	$ 511,406	$ 603,064
Less: Income allocated to participating securities									444	416	580
Net Income available to common shareholders									$ 600,999	$ 510,990	$ 602,484
Basic weighted average shares outstanding									122,788	117,200	111,012
Net Income per share — Basic	$ 0.53	$ 0.96	$ 1.95	$ 1.47	$ 0.49	$ 0.68	$ 1.83	$ 1.38	$ 4.89	$ 4.36	$ 5.43
Earnings Per Share, Diluted [Abstract]
Net Income available to common shareholders									$ 600,999	$ 510,990	$ 602,484
Effect of dilutive shares									0	0	0
Net Income available to common shareholders									$ 600,999	$ 510,990	$ 602,484
Basic weighted average shares outstanding									122,788	117,200	111,012
Dilutive shares									84	261	0
Diluted weighted average shares outstanding									122,872	117,461	111,012
Net Income per share — Diluted	$ 0.53	$ 0.96	$ 1.95	$ 1.47	$ 0.49	$ 0.68	$ 1.82	$ 1.38	$ 4.89	$ 4.35	$ 5.43