Condensed Consolidating Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2017	Feb. 29, 2016
Condensed Financial Statements Captions [Line Items]
REVENUES	$ 14,997	$ 12,654	$ 44,591	$ 37,386
OPERATING EXPENSES:
Salaries and employee benefits	5,395	4,712	16,059	13,807
Purchased transportation	3,498	2,623	10,169	7,505
Rentals and landing fees	834	744	2,426	2,121
Depreciation and amortization	762	663	2,241	1,964
Fuel	735	537	2,043	1,864
Maintenance and repairs	588	504	1,765	1,581
Other	2,160	2,007	6,432	5,399
OPERATING EXPENSES	13,972	11,790	41,135	34,241
OPERATING INCOME	1,025	864	3,456	3,145
OTHER INCOME (EXPENSE):
Interest, net	(122)	(81)	(354)	(218)
Other, net	(4)	(1)	17	(6)
INCOME BEFORE INCOME TAXES	899	782	3,119	2,921
PROVISION FOR INCOME TAXES	337	275	1,142	1,031
NET INCOME	562	507	1,977	1,890
COMPREHENSIVE INCOME	653	389	1,812	1,559
Consolidation Eliminations Member
Condensed Financial Statements Captions [Line Items]
REVENUES	(72)	(76)	(212)	(253)
OPERATING EXPENSES:
Purchased transportation	(27)	(28)	(81)	(108)
Rentals and landing fees	(1)	(1)	(4)	(4)
Other	(44)	(47)	(127)	(141)
OPERATING EXPENSES	(72)	(76)	(212)	(253)
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(521)	(593)	(2,046)	(2,110)
INCOME BEFORE INCOME TAXES	(521)	(593)	(2,046)	(2,110)
NET INCOME	(521)	(593)	(2,046)	(2,110)
COMPREHENSIVE INCOME	(521)	(593)	(2,046)	(2,110)
Parent Company Member
OPERATING EXPENSES:
Salaries and employee benefits	29	32	94	92
Rentals and landing fees	1	1	4	4
Depreciation and amortization	1		1	1
Maintenance and repairs	1		1
Intercompany charges, net	(87)	(344)	(266)	(525)
Other	55	311	166	428
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	562	507	1,977	1,890
Interest, net	(129)	(90)	(374)	(246)
Intercompany charges, net	130	95	376	257
Other, net	(1)	(5)	(2)	(11)
INCOME BEFORE INCOME TAXES	562	507	1,977	1,890
NET INCOME	562	507	1,977	1,890
COMPREHENSIVE INCOME	543	488	1,921	1,834
Guarantor Subsidiaries Member
Condensed Financial Statements Captions [Line Items]
REVENUES	11,275	10,838	33,175	31,190
OPERATING EXPENSES:
Salaries and employee benefits	4,210	4,072	12,477	11,811
Purchased transportation	2,219	2,106	6,210	5,481
Rentals and landing fees	657	660	1,902	1,843
Depreciation and amortization	649	608	1,894	1,792
Fuel	657	520	1,819	1,808
Maintenance and repairs	514	471	1,544	1,476
Intercompany charges, net	(33)	294	67	338
Other	1,428	1,344	4,230	3,901
OPERATING EXPENSES	10,301	10,075	30,143	28,450
OPERATING INCOME	974	763	3,032	2,740
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(41)	86	69	220
Interest, net	6	6	19	20
Intercompany charges, net	(79)	(105)	(224)	(264)
Other, net	(118)	(1)	(128)	(10)
INCOME BEFORE INCOME TAXES	742	749	2,768	2,706
PROVISION FOR INCOME TAXES	280	249	951	915
NET INCOME	462	500	1,817	1,791
COMPREHENSIVE INCOME	444	487	1,781	1,758
Non Guarantor Subsidiaries Member
Condensed Financial Statements Captions [Line Items]
REVENUES	3,794	1,892	11,628	6,449
OPERATING EXPENSES:
Salaries and employee benefits	1,156	608	3,488	1,904
Purchased transportation	1,306	545	4,040	2,132
Rentals and landing fees	177	84	524	278
Depreciation and amortization	112	55	346	171
Fuel	78	17	224	56
Maintenance and repairs	73	33	220	105
Intercompany charges, net	120	50	199	187
Other	721	399	2,163	1,211
OPERATING EXPENSES	3,743	1,791	11,204	6,044
OPERATING INCOME	51	101	424	405
OTHER INCOME (EXPENSE):
Interest, net	1	3	1	8
Intercompany charges, net	(51)	10	(152)	7
Other, net	115	5	147	15
INCOME BEFORE INCOME TAXES	116	119	420	435
PROVISION FOR INCOME TAXES	57	26	191	116
NET INCOME	59	93	229	319
COMPREHENSIVE INCOME	$ 187	$ 7	$ 156	$ 77