Earnings Per Common Share (Computation of Basic and Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Earnings Per Common Share [Abstract]
Net income, Basic earnings per share	$ 85,519	$ 59,466	$ 158,350	$ 124,934
Shares, Basic earnings per share	65,733	65,276	65,631	65,150
Per Share Amount, Basic earnings per share	$ 1.30	$ 0.91	$ 2.41	$ 1.92
Effect of dilutive securities:
Shares, stock options	388	305	399	326
Per Share Amount, Stock options	$ (0.01)	$ (0.01)	$ (0.01)	$ (0.01)
Shares, restricted stock	468	200	311	184
Per Share Amount, Restricted Stock	$ (0.01)	$ 0.00	$ (0.01)	$ (0.01)
Net Income, Diluted earnings per share	$ 85,519	$ 59,466	$ 158,350	$ 124,934
Diluted earnings per share, Shares	66,589	65,781	66,341	65,660
Per Share Amount, Diluted earnings per share	$ 1.28	$ 0.90	$ 2.39	$ 1.90
Outstanding stock options, excluded in the calculation of earnings per share	289	562	289	562