Guarantor and Non-Guarantor Financial Information - Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
CONDENSED CONSOLIDATING STATEMENTS OF INCOME
Operating revenues	$ 438,191	$ 885,670	$ 926,038	$ 1,946,457
Operating costs and other	396,570	654,344	845,747	1,384,760
Operating income (loss) from continuing operations	41,621	231,326	80,291	561,697
Other income (loss), net	1,452	2,619	1,924	3,228
Interest expense	(5,721)	(2,600)	(10,245)	(3,190)
Income (loss) from continuing operations before income taxes	37,352	231,345	71,970	561,735
Income tax provision	12,178	77,803	30,898	204,570
Income (loss) from continuing operations	25,174	153,542	41,072	357,165
Income (loss) from discontinued operations before income taxes	(56)	(76)	48	(91)
Income tax provision	3,913	(77)	3,913	(77)
Income (loss) from discontinued operations	(3,969)	1	(3,865)	(14)
NET INCOME	21,205	153,543	37,207	357,151
Eliminations
CONDENSED CONSOLIDATING STATEMENTS OF INCOME
Operating revenues	(16)	(24)	(33)	(40)
Operating costs and other	(402)	(1,034)	(803)	(2,066)
Operating income (loss) from continuing operations	386	1,010	770	2,026
Other income (loss), net	(386)	(1,010)	(770)	(2,026)
Equity in net income (loss) of subsidiaries	(15,247)	(159,079)	(24,599)	(371,156)
Income (loss) from continuing operations before income taxes	(15,247)	(159,079)	(24,599)	(371,156)
Income (loss) from continuing operations	(15,247)	(159,079)	(24,599)	(371,156)
NET INCOME	(15,247)	(159,079)	(24,599)	(371,156)
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF INCOME
Operating costs and other	3,156	3,059	5,861	6,080
Operating income (loss) from continuing operations	(3,156)	(3,059)	(5,861)	(6,080)
Other income (loss), net	(271)	2	(251)	3
Interest expense	(62)	(12)	(124)	(19)
Equity in net income (loss) of subsidiaries	23,065	155,706	40,614	361,065
Income (loss) from continuing operations before income taxes	19,576	152,637	34,378	354,969
Income tax provision	(1,629)	(906)	(2,829)	(2,182)
Income (loss) from continuing operations	21,205	153,543	37,207	357,151
NET INCOME	21,205	153,543	37,207	357,151
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF INCOME
Operating revenues	382,747	774,896	788,284	1,727,005
Operating costs and other	331,998	548,083	688,749	1,177,121
Operating income (loss) from continuing operations	50,749	226,813	99,535	549,884
Other income (loss), net	734	3,738	1,390	4,659
Interest expense	(5,272)		(9,990)	(103)
Interest income (expense)		(121)
Equity in net income (loss) of subsidiaries	(7,818)	3,373	(16,015)	10,091
Income (loss) from continuing operations before income taxes	38,393	233,803	74,920	564,531
Income tax provision	15,657	78,923	34,884	205,045
Income (loss) from continuing operations	22,736	154,880	40,036	359,486
NET INCOME	22,736	154,880	40,036	359,486
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF INCOME
Operating revenues	55,460	110,798	137,787	219,492
Operating costs and other	61,818	104,236	151,940	203,625
Operating income (loss) from continuing operations	(6,358)	6,562	(14,153)	15,867
Other income (loss), net	1,375	(111)	1,555	592
Interest expense	(387)	(2,467)	(131)	(3,068)
Income (loss) from continuing operations before income taxes	(5,370)	3,984	(12,729)	13,391
Income tax provision	(1,850)	(214)	(1,157)	1,707
Income (loss) from continuing operations	(3,520)	4,198	(11,572)	11,684
Income (loss) from discontinued operations before income taxes	(56)	(76)	48	(91)
Income tax provision	3,913	(77)	3,913	(77)
Income (loss) from discontinued operations	(3,969)	1	(3,865)	(14)
NET INCOME	$ (7,489)	$ 4,199	$ (15,437)	$ 11,670