Basis of Presentation (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Oct. 01, 2013	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2015
Basis of presentation
Cumulative effect of an accounting change to retained earnings net of income tax	$ 1,600
Operating revenues			$ 487,847	$ 1,060,787
Operating costs, excluding depreciation			276,644	559,463
Net income			$ 16,002	$ 203,608
Basic earnings per common share (in dollars per share)			$ 0.15	$ 1.86
Total assets			$ 7,130,274		$ 7,147,242
Total liabilities					2,251,396
Total shareholders' equity			4,830,293		4,895,846
Early termination revenue			28,900	$ 23,400
Abandonments			500	2,200
Foreign currency losses			8,500	(1,500)
Previous Accounting Guidance [Member]
Basis of presentation
Net income			5,400
Foreign currency losses			$ (6,300)
Subsequent Event.
Basis of presentation
Early termination revenue		$ 78,000
As Reported
Basis of presentation
Operating revenues				1,056,585
Operating costs, excluding depreciation				554,243
Net income				$ 203,042
Basic earnings per common share (in dollars per share)				$ 1.85
Total assets					7,152,012
Total liabilities					2,254,560
Total shareholders' equity					4,897,452
Adjustments
Basis of presentation
Operating revenues				$ 4,202
Operating costs, excluding depreciation				5,220
Net income				$ 566
Basic earnings per common share (in dollars per share)				$ 0.01
Total assets					(4,770)
Total liabilities					(3,164)
Total shareholders' equity					$ (1,606)