Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Net Income
Net income	$ 122,437	$ 122,669	$ 161,518	$ 168,887
Stock options	0	0	0	0
Restricted stock and performance units	0	0	0	0
Diluted earnings per share	$ 122,437	$ 122,669	$ 161,518	$ 168,887
Shares
Basic earnings per share (in shares)	64,781,000	64,620,000	64,739,000	64,513,000
Stock options (in shares)	0	64,000	0	80,000
Restricted stock (in shares)	44,000	364,000	131,000	438,000
Diluted earnings per share (in shares)	64,825,000	65,048,000	64,870,000	65,031,000
Per Share Amount
Basic earnings per share (in dollars per share)	$ 1.89	$ 1.90	$ 2.49	$ 2.62
Stock options (in dollars per share)	0.00	0.00	0.00	0.00
Restricted stock (in dollars per share)	0.00	(0.01)	0.00	(0.02)
Diluted earnings per share (in dollars per share)	$ 1.89	$ 1.89	$ 2.49	$ 2.60
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding stock options, excluded in the calculation of earnings per share	2,090,399	1,127,000	2,085,649	864,000