Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
Basic and Diluted Earnings Per Share, Continuing Operations [Abstract]
Net Income	$ 214,888	$ 157,646	$ 178,992	$ 314,132	$ 372,534	$ 493,124
Income allocated to participating securities	170		161		301	459
Income available to common shareholders	$ 214,718		$ 178,831		$ 372,233	$ 492,665
Basic weighted average shares outstanding	117,581		111,706		115,690	110,135
Basic net income per share	$ 1.83		$ 1.60		$ 3.22	$ 4.47
Effect of dilutive shares	$ 0		$ 0		$ 0	$ 0
Income available to common shareholders	$ 214,718		$ 178,831		$ 372,233	$ 492,665
Additional dilutive shares	175		0		104	0
Diluted weighted average shares outstanding	117,756		111,706		115,794	110,135
Diluted net income per share	$ 1.82		$ 1.60		$ 3.21	$ 4.47