Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2020	Jun. 30, 2019
Basic Earnings Per Share
Net income	$ 117,791	$ 239,646	$ 178,673	$ 80,466	$ 214,888	$ 157,646	$ 536,110	$ 453,000
Less: Income allocated to participating securities	88			64			408	386
Income available to common shareholders	$ 117,703			$ 80,402			$ 535,702	$ 452,614
Basic weighted average shares outstanding (in shares)	123,026			118,075			122,352	116,485
Net income per share - Basic (USD per share)	$ 0.96			$ 0.68			$ 4.38	$ 3.89
Diluted Earnings Per Share
Income available to common shareholders	$ 117,703			$ 80,402			$ 535,702	$ 452,614
Effect of dilutive shares	0			0			0	0
Income available to common shareholders	$ 117,703			$ 80,402			$ 535,702	$ 452,614
Basic weighted average shares outstanding (in shares)	123,026			118,075			122,352	116,485
Dilutive shares (in shares)	6			355			111	188
Diluted weighted average shares outstanding (in shares)	123,032			118,430			122,463	116,673
Net income per share - Diluted (USD per share)	$ 0.96			$ 0.68			$ 4.37	$ 3.88