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				6 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2021	Mar. 31, 2020
OPERATING EXPENSES
Depreciation and amortization	$ 26,848		$ 27,516		$ 54,210	$ 52,153
Total operating expenses	600,979		541,131		943,804	1,051,549
Operating income	201,208		98,483		312,688	203,101
Income before income taxes and equity in earnings of affiliates	186,062		86,541		281,873	175,375
Income tax provision (benefit)	39,057		16,284		56,498	32,755
Net income	$ 149,809	$ 81,045	$ 73,846	$ 75,752	$ 230,854	$ 149,598
EARNINGS PER COMMON SHARE
Basic (usd per share)	$ 1.56		$ 0.78		$ 2.40	$ 1.60
Diluted (usd per share)	$ 1.55		$ 0.77		$ 2.39	$ 1.59
As Previously Reported
OPERATING EXPENSES
Depreciation and amortization			$ 30,784			$ 58,542
Total operating expenses			544,399			1,057,938
Operating income			95,215			196,712
Income before income taxes and equity in earnings of affiliates			83,273			168,986
Income tax provision (benefit)			(1,643)			(1,902)
Net income			$ 88,505	89,361		$ 177,866
EARNINGS PER COMMON SHARE
Basic (usd per share)			$ 0.93			$ 1.90
Diluted (usd per share)			$ 0.92			$ 1.89
Effect of Change
OPERATING EXPENSES
Depreciation and amortization			$ (3,268)			$ (6,389)
Total operating expenses			(3,268)			(6,389)
Operating income			3,268			6,389
Income before income taxes and equity in earnings of affiliates			3,268			6,389
Income tax provision (benefit)			17,927			34,657
Net income			$ (14,659)	$ (13,609)		$ (28,268)
EARNINGS PER COMMON SHARE
Basic (usd per share)			$ (0.15)			$ (0.30)
Diluted (usd per share)			$ (0.15)			$ (0.30)