Long-term Purchase Commitments (Details) $ in Millions	Jun. 30, 2016 USD ($) aircraft
Long-term Purchase Commitment [Line Items]
Capital Leases, Future Minimum Payments Due, Next Twelve Months | $	$ 15
Capital Leases, Future Minimum Payments Due in Two Years | $	29
Capital Leases, Future Minimum Payments Due in Three Years | $	29
Capital Leases, Future Minimum Payments Due in Four Years | $	28
Capital Leases, Future Minimum Payments Due in Five Years | $	27
Capital Leases, Future Minimum Payments Due Thereafter | $	$ 176
Firm 737 NG 800 (in units)	89
Options - 737 NG (in units)	18
Firm - 737 MAX (in units)	30
Firm - 738 MAX (in units)	170	[1]
Options - 738 MAX (in units)	191	[1]
Additional - 700 A/C (in units)	41
Total	539
2016
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	38
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	0
Options - 738 MAX (in units)	0
Additional - 700 A/C (in units)	23
Total	61	[2]
2017
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	39
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	14
Options - 738 MAX (in units)	0
Additional - 700 A/C (in units)	14
Total	67
2018
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	12
Options - 737 NG (in units)	18
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	13
Options - 738 MAX (in units)	0
Additional - 700 A/C (in units)	4
Total	47
2019
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	15
Firm - 738 MAX (in units)	0
Options - 738 MAX (in units)	5
Additional - 700 A/C (in units)	0
Total	20
2020
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	14
Firm - 738 MAX (in units)	0
Options - 738 MAX (in units)	8
Additional - 700 A/C (in units)	0
Total	22
2021
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	1
Firm - 738 MAX (in units)	13
Options - 738 MAX (in units)	18
Additional - 700 A/C (in units)	0
Total	32
2022
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	15
Options - 738 MAX (in units)	19
Additional - 700 A/C (in units)	0
Total	34
2023
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	34
Options - 738 MAX (in units)	23
Additional - 700 A/C (in units)	0
Total	57
2024
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	41
Options - 738 MAX (in units)	23
Additional - 700 A/C (in units)	0
Total	64
2025
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	40
Options - 738 MAX (in units)	36
Additional - 700 A/C (in units)	0
Total	76
2026
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	0
Options - 738 MAX (in units)	36
Additional - 700 A/C (in units)	0
Total	36
2027
Long-term Purchase Commitment [Line Items]
Firm 737 NG 800 (in units)	0
Options - 737 NG (in units)	0
Firm - 737 MAX (in units)	0
Firm - 738 MAX (in units)	0
Options - 738 MAX (in units)	23
Additional - 700 A/C (in units)	0
Total	23
Aircraft [Member]
Long-term Purchase Commitment [Line Items]
Committed expenditures, 2016 | $	$ 592
Committed expenditures, 2017 | $	1,100
Committed expenditures, 2018 | $	681
Committed expenditures, 2019 | $	614
Committed expenditures, 2020 | $	821
Committed expenditures, 2021 and beyond | $	$ 6,100

[1] The Company has flexibility to substitute 737-7 in lieu of 737-8 aircraft beginning in 2019.

[2] Includes 14 737-800s and 19 737-700s delivered as of June 30, 2016. The 19 737-700s are classified as capital leases and have increased the Company's future capital lease obligations by $15 million remaining in 2016, $29 million in 2017, $29 million in 2018, $28 million in 2019, $27 million in 2020, and $176 million thereafter.