Commitments and Contingencies - MD-90 Fleet Retirement (Details) $ in Millions	3 Months Ended
	Jun. 30, 2019 USD ($) aircraft
MD-90 Fleet Retirement
Loss Contingencies [Line Items]
Accelerated depreciation related to permanent fleet retirement | $	$ 91
MD-90
Loss Contingencies [Line Items]
Time ahead of plan for fleet retirement	2 years
Number of aircraft permanently retired | aircraft	31