Condensed Consolidating Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2018	Nov. 30, 2017	Nov. 30, 2018	Nov. 30, 2017
Condensed Statement Of Income Captions [Line Items]
REVENUES	$ 17,824	$ 16,313	$ 34,876	$ 31,610
OPERATING EXPENSES:
Salaries and employee benefits	6,260	5,889	12,520	11,553
Purchased transportation	$ 4,346	$ 3,840	$ 8,313	$ 7,285
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 836	$ 835	$ 1,659	$ 1,653
Depreciation and amortization	828	756	1,636	1,507
Fuel	1,052	818	2,038	1,521
Maintenance and repairs	751	665	1,486	1,340
Other	2,583	2,395	4,985	4,665
OPERATING EXPENSES	16,656	15,198	32,637	29,524
OPERATING INCOME	1,168	1,115	2,239	2,086
OTHER INCOME (EXPENSE):
Interest, net	(129)	(124)	(241)	(238)
Other retirement plans income	158	147	316	293
Other, net	(20)	1	(36)	(20)
INCOME BEFORE INCOME TAXES	1,177	1,139	2,278	2,121
Provision for income taxes	242	364	508	750
NET INCOME	935	775	1,770	1,371
COMPREHENSIVE INCOME	881	666	1,531	1,352
Consolidation Eliminations [Member]
Condensed Statement Of Income Captions [Line Items]
REVENUES	(100)	(93)	(202)	(217)
OPERATING EXPENSES:
Purchased transportation	$ (35)	$ (50)	$ (83)	$ (132)
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ (1)	$ (3)	$ (3)	$ (4)
Other	(72)	(40)	(125)	(81)
OPERATING EXPENSES	(108)	(93)	(211)	(217)
OPERATING INCOME	8		9
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(983)	(815)	(1,898)	(1,408)
Other, net	(9)		(10)
INCOME BEFORE INCOME TAXES	(984)	(815)	(1,899)	(1,408)
NET INCOME	(984)	(815)	(1,899)	(1,408)
COMPREHENSIVE INCOME	(983)	(815)	(1,899)	(1,408)
Parent Company [Member] | Reportable Legal Entities [Member]
OPERATING EXPENSES:
Salaries and employee benefits	$ 34	$ 35	$ 82	$ 73
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 1	$ 1	$ 3	$ 2
Maintenance and repairs			1
Intercompany charges, net	(152)	(95)	(264)	(211)
Other	117	59	178	136
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	935	775	1,770	1,371
Interest, net	(180)	(130)	(338)	(259)
Intercompany charges, net	162	132	304	263
Other, net	18	(2)	34	(4)
INCOME BEFORE INCOME TAXES	935	775	1,770	1,371
NET INCOME	935	775	1,770	1,371
COMPREHENSIVE INCOME	912	756	1,730	1,334
Guarantor Subsidiaries [Member[ | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
REVENUES	12,874	12,044	25,241	23,611
OPERATING EXPENSES:
Salaries and employee benefits	4,797	4,580	9,580	8,949
Purchased transportation	$ 2,650	$ 2,314	$ 5,032	$ 4,377
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 640	$ 640	$ 1,271	$ 1,267
Depreciation and amortization	709	649	1,402	1,288
Fuel	968	746	1,871	1,383
Maintenance and repairs	655	584	1,301	1,186
Intercompany charges, net	(149)	1	(375)	115
Other	1,675	1,592	3,221	3,068
OPERATING EXPENSES	11,945	11,106	23,303	21,633
OPERATING INCOME	929	938	1,938	1,978
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	48	40	128	37
Interest, net	63	11	122	24
Other retirement plans income	155	141	311	283
Intercompany charges, net	(124)	(70)	(246)	(140)
Other, net	(22)	(8)	(76)	(16)
INCOME BEFORE INCOME TAXES	1,049	1,052	2,177	2,166
Provision for income taxes	217	225	432	624
NET INCOME	832	827	1,745	1,542
COMPREHENSIVE INCOME	819	817	1,833	1,536
Non Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
REVENUES	5,050	4,362	9,837	8,216
OPERATING EXPENSES:
Salaries and employee benefits	1,429	1,274	2,858	2,531
Purchased transportation	$ 1,731	$ 1,576	$ 3,364	$ 3,040
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 196	$ 197	$ 388	$ 388
Depreciation and amortization	119	107	234	219
Fuel	84	72	167	138
Maintenance and repairs	96	81	184	154
Intercompany charges, net	301	94	639	96
Other	863	784	1,711	1,542
OPERATING EXPENSES	4,819	4,185	9,545	8,108
OPERATING INCOME	231	177	292	108
OTHER INCOME (EXPENSE):
Interest, net	(12)	(5)	(25)	(3)
Other retirement plans income	3	6	5	10
Intercompany charges, net	(38)	(62)	(58)	(123)
Other, net	(7)	11	16
INCOME BEFORE INCOME TAXES	177	127	230	(8)
Provision for income taxes	25	139	76	126
NET INCOME	152	(12)	154	(134)
COMPREHENSIVE INCOME	$ 133	$ (92)	$ (133)	$ (110)