Quarterly Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Quarterly Information
Total revenues	$ 258,352	$ 228,964	$ 256,278	$ 248,873	$ 234,085	$ 203,152	$ 213,254	$ 218,826	$ 992,467	$ 869,317	$ 733,045
Net revenues	163,318	145,260	161,622	154,028	146,759	122,997	130,950	134,232	624,228	534,938	447,479
Net Income	$ 38,572	$ 28,523	$ 37,261	$ 39,352	$ 36,365	$ 26,527	$ 31,838	$ 27,429	$ 143,708	$ 122,159	$ 97,294
Net income per common share:
Basic (in dollars per share)	$ 0.75	$ 0.56	$ 0.73	$ 0.78	$ 0.73	$ 0.53	$ 0.64	$ 0.55	$ 2.82	$ 2.45	$ 1.98
Diluted (in dollars per share)	$ 0.75	$ 0.56	$ 0.73	$ 0.78	$ 0.72	$ 0.53	$ 0.63	$ 0.55	$ 2.81	$ 2.43	$ 1.96