Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2021	Jun. 30, 2020
Basic Earnings Per Share
Net income	$ 102,411	$ 296,754	$ 217,678	$ 117,791	$ 239,646	$ 178,673	$ 616,843	$ 536,110
Less: Income allocated to participating securities	70			88			440	408
Income available to common shareholders	$ 102,341			$ 117,703			$ 616,403	$ 535,702
Basic weighted average shares outstanding (in shares)	131,358			123,026			129,185	122,352
Net income per share - Basic (USD per share)	$ 0.78			$ 0.96			$ 4.77	$ 4.38
Diluted Earnings Per Share
Income available to common shareholders	$ 102,341			$ 117,703			$ 616,403	$ 535,702
Effect of dilutive shares	0			0			0	0
Income available to common shareholders	$ 102,341			$ 117,703			$ 616,403	$ 535,702
Basic weighted average shares outstanding (in shares)	131,358			123,026			129,185	122,352
Dilutive shares (in shares)	128			6			44	111
Diluted weighted average shares outstanding (in shares)	131,486			123,032			129,229	122,463
Net income per share - Diluted (USD per share)	$ 0.78			$ 0.96			$ 4.77	$ 4.37