Earnings Per Common Share - Narrative (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Net Income
Basic earnings per share	$ 122,669	$ 73,300	$ 168,887	$ 156,697
Stock options	0	0	0	0
Restricted stock and performance units	0	0	0	0
Diluted earnings per share	$ 122,669	$ 73,300	$ 168,887	$ 156,697
Shares
Basic earnings per share (in shares)	64,620,000	64,238,000	64,513,000	64,175,000
Stock options (in shares)	64,000	227,000	80,000	267,000
Restricted stock (in shares)	364,000	566,000	438,000	586,000
Diluted earnings per share (in shares)	65,048,000	65,031,000	65,031,000	65,028,000
Per Share Amount
Basic earnings per share (in dollars per share)	$ 1.90	$ 1.14	$ 2.62	$ 2.44
Stock options (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ (0.01)
Restricted stock (in dollars per share)	$ (0.01)	$ (0.01)	$ (0.02)	$ (0.02)
Diluted earnings per share (in dollars per share)	$ 1.89	$ 1.13	$ 2.60	$ 2.41
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding stock options, excluded in the calculation of earnings per share	1,127,000	252,000	864,000	252,000