CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	[1]	$ 73,573	$ 140,266	$ 159,821	$ 263,965
Other comprehensive income (loss), net of tax
Unrealized loss on investments in equity securities, net of tax of $0, $7,366, $0 and $8,217, respectively		0	(20,401)	0	(22,691)
Reclassifications of losses to net income on investments in equity securities, net of tax of $0, $(3,036), $0 and $(858), respectively		0	14,801	0	11,647
Adjustment to postemployment benefit obligation, net of tax of $(119), $(104), $(215) and $(240), respectively		305	272	539	512
Other comprehensive income (loss)		305	(5,328)	539	(10,532)
Comprehensive income		$ 73,878	$ 134,938	$ 160,360	$ 253,433

[1] Includes income tax benefit related to the Tax Act of $967,000 and $58.5 million, for the three and six months ended March 31, 2018, respectively.