SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - IMPACT OF ASC 740 ON UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
OPERATING EXPENSES
Depreciation and amortization	$ 27,362	$ 24,637
Total operating expenses	342,825	510,418
Operating income	111,480	104,618
Income before income taxes and equity in earnings of affiliates	95,811	88,834
Income tax provision	17,441	16,471
Net income	$ 81,045	$ 75,752
EARNINGS PER COMMON SHARE
Basic (usd per share)	$ 0.84	$ 0.82
Diluted (usd per share)	$ 0.84	$ 0.82
As Previously Reported
OPERATING EXPENSES
Depreciation and amortization		$ 27,758
Total operating expenses		513,539
Operating income		101,497
Income before income taxes and equity in earnings of affiliates		85,713
Income tax provision		(259)
Net income		$ 89,361
EARNINGS PER COMMON SHARE
Basic (usd per share)		$ 0.97
Diluted (usd per share)		$ 0.97
Effect of Change
OPERATING EXPENSES
Depreciation and amortization		$ (3,121)
Total operating expenses		(3,121)
Operating income		3,121
Income before income taxes and equity in earnings of affiliates		3,121
Income tax provision		16,730
Net income		$ (13,609)
EARNINGS PER COMMON SHARE
Basic (usd per share)		$ (0.15)
Diluted (usd per share)		$ (0.15)