Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Basic Earnings per Share
Net income	$ 34,240	$ 71,193	$ 141,810	$ 102,861	$ 23,515	$ 56,281	$ 137,684	$ 97,595	$ 350,104	$ 315,075	$ 289,817
Less: Income allocated to participating securities									546	626	711
Net Income available to common shareholders									$ 349,558	$ 314,449	$ 289,106
Basic weighted average shares outstanding (shares)									103,524	101,892	97,606
Income per share - basic (usd per share)									$ 3.38	$ 3.09	$ 2.96
Diluted Earnings per Share
Effect of dilutive stock options and other shares									$ 0	$ 0	$ 0
Net Income available to common shareholders									$ 349,558	$ 314,449	$ 289,106
Additional dilutive stock options and other shares									0	0	2
Diluted weighted average shares outstanding (shares)									103,524	101,892	97,608
Diluted net income per share	$ 0.33	$ 0.69	$ 1.38	$ 1.00	$ 0.23	$ 0.55	$ 1.35	$ 0.96	$ 3.38	$ 3.09	$ 2.96