Condensed Consolidating Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2017	Nov. 30, 2016	Nov. 30, 2017	Nov. 30, 2016
Condensed Financial Statements Captions [Line Items]
REVENUES	$ 16,313	$ 14,931	$ 31,610	$ 29,594
OPERATING EXPENSES:
Salaries and employee benefits	5,742	5,353	11,260	10,664
Purchased transportation	3,840	3,431	7,285	6,671
Rentals and landing fees	835	802	1,653	1,592
Depreciation and amortization	756	740	1,507	1,479
Fuel	818	658	1,521	1,308
Maintenance and repairs	665	579	1,340	1,177
Other	2,395	2,201	4,665	4,272
OPERATING EXPENSES	15,051	13,764	29,231	27,163
OPERATING INCOME	1,262	1,167	2,379	2,431
OTHER INCOME (EXPENSE):
Interest, net	(124)	(119)	(238)	(232)
Other, net	1	30	(20)	21
INCOME BEFORE INCOME TAXES	1,139	1,078	2,121	2,220
PROVISION FOR INCOME TAXES	364	378	750	805
NET INCOME	775	700	1,371	1,415
COMPREHENSIVE INCOME	666	451	1,352	1,159
Consolidation Eliminations [Member]
Condensed Financial Statements Captions [Line Items]
REVENUES	(93)	(70)	(217)	(140)
OPERATING EXPENSES:
Purchased transportation	(50)	(26)	(132)	(54)
Rentals and landing fees	(3)	(2)	(4)	(3)
Other	(40)	(42)	(81)	(83)
OPERATING EXPENSES	(93)	(70)	(217)	(140)
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(815)	(754)	(1,408)	(1,525)
INCOME BEFORE INCOME TAXES	(815)	(754)	(1,408)	(1,525)
NET INCOME	(815)	(754)	(1,408)	(1,525)
COMPREHENSIVE INCOME	(815)	(754)	(1,408)	(1,525)
Parent Company [Member] | Reportable Legal Entities [Member]
OPERATING EXPENSES:
Salaries and employee benefits	35	29	73	65
Rentals and landing fees	1	2	2	3
Intercompany charges, net	(95)	(89)	(211)	(179)
Other	59	58	136	111
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	775	700	1,371	1,415
Interest, net	(130)	(123)	(259)	(245)
Intercompany charges, net	132	124	263	246
Other, net	(2)	(1)	(4)	(1)
INCOME BEFORE INCOME TAXES	775	700	1,371	1,415
NET INCOME	775	700	1,371	1,415
COMPREHENSIVE INCOME	756	682	1,334	1,378
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements Captions [Line Items]
REVENUES	12,044	10,997	23,611	21,900
OPERATING EXPENSES:
Salaries and employee benefits	4,439	4,161	8,666	8,267
Purchased transportation	2,314	2,074	4,377	3,991
Rentals and landing fees	640	625	1,267	1,245
Depreciation and amortization	649	634	1,288	1,245
Fuel	746	584	1,383	1,162
Maintenance and repairs	584	504	1,186	1,030
Intercompany charges, net		38	113	100
Other	1,592	1,429	3,068	2,802
OPERATING EXPENSES	10,964	10,049	21,348	19,842
OPERATING INCOME	1,080	948	2,263	2,058
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	40	54	37	110
Interest, net	11	4	24	13
Intercompany charges, net	(71)	(64)	(142)	(145)
Other, net	(8)	(5)	(16)	(10)
INCOME BEFORE INCOME TAXES	1,052	937	2,166	2,026
PROVISION FOR INCOME TAXES	225	291	624	671
NET INCOME	827	646	1,542	1,355
COMPREHENSIVE INCOME	817	635	1,536	1,337
Non Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements Captions [Line Items]
REVENUES	4,362	4,004	8,216	7,834
OPERATING EXPENSES:
Salaries and employee benefits	1,268	1,163	2,521	2,332
Purchased transportation	1,576	1,383	3,040	2,734
Rentals and landing fees	197	177	388	347
Depreciation and amortization	107	106	219	234
Fuel	72	74	138	146
Maintenance and repairs	81	75	154	147
Intercompany charges, net	95	51	98	79
Other	784	756	1,542	1,442
OPERATING EXPENSES	4,180	3,785	8,100	7,461
OPERATING INCOME	182	219	116	373
OTHER INCOME (EXPENSE):
Interest, net	(5)		(3)
Intercompany charges, net	(61)	(60)	(121)	(101)
Other, net	11	36		32
INCOME BEFORE INCOME TAXES	127	195	(8)	304
PROVISION FOR INCOME TAXES	139	87	126	134
NET INCOME	(12)	108	(134)	170
COMPREHENSIVE INCOME	$ (92)	$ (112)	$ (110)	$ (31)