Employee Benefit Plans (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended			6 Months Ended
	Apr. 01, 2017	Mar. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of Net Period Benefit Cost
Service cost				$ 3,813	$ 3,713	$ 7,626	$ 7,426
Other cost:
Interest cost				7,259	7,582	14,518	15,164
Expected return on plan assets				(4,796)	(4,472)	(9,592)	(8,944)
Recognized net actuarial loss				2,287	1,972	4,574	3,944
Total other components of the net periodic benefit cost				4,750	5,082	9,500	10,164
Net periodic benefit cost				8,563	8,795	17,126	17,590
Employer contribution				$ 8,000	$ 11,000	$ 14,400	$ 11,300
Air Line Pilots Association
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Ratified agreement, term	63 months	63 months
Forecast | Air Line Pilots Association
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Payment to settle plan obligations			$ 102,000
Forecast | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Payment to settle plan obligations			17,000
Forecast | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Payment to settle plan obligations			$ 22,000