Condensed Consolidating Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2018	Aug. 31, 2017
Condensed Statement Of Income Captions [Line Items]
REVENUES	$ 17,052	$ 15,297
OPERATING EXPENSES:
Salaries and employee benefits	6,260	5,664
Purchased transportation	$ 3,967	$ 3,445
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 823	$ 818
Depreciation and amortization	808	751
Fuel	986	703
Maintenance and repairs	735	675
Other	2,402	2,270
OPERATING EXPENSES	15,981	14,326
OPERATING INCOME	1,071	971
OTHER INCOME (EXPENSE):
Interest, net	(112)	(114)
Other retirement plans income	158	146
Other, net	(16)	(21)
INCOME BEFORE INCOME TAXES	1,101	982
Provision for income taxes	266	386
NET INCOME	835	596
COMPREHENSIVE INCOME	650	686
Consolidation Eliminations [Member]
Condensed Statement Of Income Captions [Line Items]
REVENUES	(102)	(124)
OPERATING EXPENSES:
Purchased transportation	$ (47)	$ (82)
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ (1)	$ (1)
Other	(54)	(41)
OPERATING EXPENSES	(102)	(124)
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(916)	(593)
Other retirement plans income	(48)
Intercompany charges, net	32
INCOME BEFORE INCOME TAXES	(932)	(593)
NET INCOME	(932)	(593)
COMPREHENSIVE INCOME	(933)	(593)
Parent Company [Member] | Reportable Legal Entities [Member]
OPERATING EXPENSES:
Salaries and employee benefits	$ 48	$ 38
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 1	$ 1
Intercompany charges, net	(111)	(116)
Other	62	77
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	835	596
Interest, net	(158)	(129)
Intercompany charges, net	143	131
Other, net	(3)	(2)
INCOME BEFORE INCOME TAXES	817	596
NET INCOME	817	596
COMPREHENSIVE INCOME	800	578
Guarantor Subsidiaries [Member[ | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
REVENUES	12,367	11,567
OPERATING EXPENSES:
Salaries and employee benefits	4,783	4,368
Purchased transportation	$ 2,380	$ 2,063
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 631	$ 627
Depreciation and amortization	693	639
Fuel	902	637
Maintenance and repairs	646	602
Intercompany charges, net	(226)	114
Other	1,546	1,476
OPERATING EXPENSES	11,355	10,526
OPERATING INCOME	1,012	1,041
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	81	(3)
Interest, net	59	13
Other retirement plans income	193	141
Intercompany charges, net	(154)	(70)
Other, net	327	(8)
INCOME BEFORE INCOME TAXES	1,518	1,114
Provision for income taxes	215	399
NET INCOME	1,303	715
COMPREHENSIVE INCOME	1,401	719
Non Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
Condensed Statement Of Income Captions [Line Items]
REVENUES	4,787	3,854
OPERATING EXPENSES:
Salaries and employee benefits	1,429	1,258
Purchased transportation	$ 1,634	$ 1,464
Type of cost, good or service [extensible list]	us-gaap:ShippingAndHandlingMember	us-gaap:ShippingAndHandlingMember
Rentals and landing fees	$ 192	$ 191
Depreciation and amortization	115	112
Fuel	84	66
Maintenance and repairs	89	73
Intercompany charges, net	337	2
Other	848	758
OPERATING EXPENSES	4,728	3,924
OPERATING INCOME	59	(70)
OTHER INCOME (EXPENSE):
Interest, net	(13)	2
Other retirement plans income	13	5
Intercompany charges, net	(21)	(61)
Other, net	(340)	(11)
INCOME BEFORE INCOME TAXES	(302)	(135)
Provision for income taxes	51	(13)
NET INCOME	(353)	(122)
COMPREHENSIVE INCOME	$ (618)	$ (18)