Basis of Presentation (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended			6 Months Ended
	Oct. 01, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Basis of presentation
Cumulative effect of an accounting change to retained earnings net of income tax	$ 1,600
Net income from continuing operations		$ 25,174		$ 153,542	$ 41,072	$ 357,165
Adjustment for loss from discontinued operations		3,969		(1)	3,865	14
Operating revenues		438,191		885,670	926,038	1,946,457
Operating costs, excluding depreciation		221,611		467,099	498,255	1,026,562
Net income		$ 21,205		$ 153,543	$ 37,207	$ 357,151
Diluted earnings per common share (in dollars per share)		$ 0.19		$ 1.41	$ 0.33	$ 3.27
Total assets		$ 7,025,105			$ 7,025,105		$ 7,147,242
Total liabilities							2,251,396
Total shareholders' equity		4,782,100			4,782,100		4,895,846
Early termination revenue		79,600		$ 72,400	108,400	$ 95,800
Abandonments		300		10,100	800	12,300
Foreign currency gains losses		(200)		300	8,300	1,700
Previous Accounting Guidance
Basis of presentation
Net income from continuing operations		6,300			(900)
Adjustment for loss from discontinued operations		$ 4,000			$ 4,000
Foreign currency gains losses			$ 6,100
As Reported
Basis of presentation
Operating revenues				883,052		1,939,637
Operating costs, excluding depreciation				469,328		1,023,571
Net income				$ 149,537		$ 352,579
Diluted earnings per common share (in dollars per share)				$ 1.37		$ 3.23
Total assets							7,152,012
Total liabilities							2,254,560
Total shareholders' equity							4,897,452
Adjustments
Basis of presentation
Operating revenues				$ 2,618		$ (6,820)
Operating costs, excluding depreciation				(2,229)		(2,991)
Net income				$ 4,006		$ 4,572
Diluted earnings per common share (in dollars per share)				$ 0.04		$ 0.04
Total assets							(4,770)
Total liabilities							(3,164)
Total shareholders' equity							$ (1,606)