Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019
Basic Earnings Per Share
Net income	$ 239,646	$ 178,673	$ 214,888	$ 157,646	$ 418,319	$ 372,534
Less: Income allocated to participating securities	178		170		314	301
Income available to common shareholders	$ 239,468		$ 214,718		$ 418,005	$ 372,233
Basic weighted average shares outstanding (in shares)	122,916		117,581		122,015	115,690
Net income per share - Basic (USD per share)	$ 1.95		$ 1.83		$ 3.43	$ 3.22
Diluted Earnings Per Share
Income available to common shareholders	$ 239,468		$ 214,718		$ 418,005	$ 372,233
Effect of dilutive shares	0		0		0	0
Income available to common shareholders	$ 239,468		$ 214,718		$ 418,005	$ 372,233
Basic weighted average shares outstanding (in shares)	122,916		117,581		122,015	115,690
Dilutive shares (in shares)	81		175		164	104
Diluted weighted average shares outstanding (in shares)	122,997		117,756		122,179	115,794
Net income per share - Diluted (USD per share)	$ 1.95		$ 1.82		$ 3.42	$ 3.21