Commitments and Contingengies - Long-term Purchase Commitments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) aircraft
Aircraft [Member]
Long-term Purchase Commitment [Line Items]
Committed expenditures, 2018 | $	$ 874
Committed expenditures, 2019 | $	666
Committed expenditures, 2020 | $	1,100
Committed expenditures, 2021 | $	1,300
Committed expenditures, 2022 | $	877
Committed expenditures, 2023 and beyond | $	$ 5,200
B-737-Max [Member]
Long-term Purchase Commitment [Line Items]
Property, Plant and Equipment, Number of Aircraft Purchased	13
B-737-800
Long-term Purchase Commitment [Line Items]
Property, Plant and Equipment, Number of Aircraft Purchased	39
Long-term Purchase Commitment, Minimum Quantity Required	26
B-737-700
Long-term Purchase Commitment [Line Items]
Property, Plant and Equipment, Number of Aircraft Purchased	18
B-737-8 Max
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment, Minimum Quantity Required	197
Options - 737MAX (in units)	155
B-737-7 Max
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment, Minimum Quantity Required	30