Condensed Consolidating Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2017	Aug. 31, 2016
Condensed Financial Statements Captions [Line Items]
REVENUES	$ 15,297	$ 14,663
OPERATING EXPENSES:
Salaries and employee benefits	5,518	5,311
Purchased transportation	3,445	3,240
Rentals and landing fees	818	790
Depreciation and amortization	751	739
Fuel	703	650
Maintenance and repairs	675	598
Other	2,270	2,071
OPERATING EXPENSES	14,180	13,399
OPERATING INCOME	1,117	1,264
OTHER INCOME (EXPENSE):
Interest, net	(114)	(113)
Other, net	(21)	(9)
INCOME BEFORE INCOME TAXES	982	1,142
PROVISION FOR INCOME TAXES	386	427
NET INCOME	596	715
COMPREHENSIVE INCOME	686	708
Consolidation Eliminations [Member]
Condensed Financial Statements Captions [Line Items]
REVENUES	(124)	(70)
OPERATING EXPENSES:
Purchased transportation	(82)	(28)
Rentals and landing fees	(1)	(1)
Other	(41)	(41)
OPERATING EXPENSES	(124)	(70)
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(593)	(771)
INCOME BEFORE INCOME TAXES	(593)	(771)
NET INCOME	(593)	(771)
COMPREHENSIVE INCOME	(593)	(771)
Parent Company [Member]
OPERATING EXPENSES:
Salaries and employee benefits	38	36
Rentals and landing fees	1	1
Intercompany charges, net	(116)	(90)
Other	77	53
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	596	715
Interest, net	(129)	(122)
Intercompany charges, net	131	122
Other, net	(2)
INCOME BEFORE INCOME TAXES	596	715
NET INCOME	596	715
COMPREHENSIVE INCOME	578	696
Guarantor Subsidiaries [Member]
Condensed Financial Statements Captions [Line Items]
REVENUES	11,567	10,903
OPERATING EXPENSES:
Salaries and employee benefits	4,227	4,106
Purchased transportation	2,063	1,917
Rentals and landing fees	627	620
Depreciation and amortization	639	611
Fuel	637	578
Maintenance and repairs	602	526
Intercompany charges, net	113	62
Other	1,476	1,373
OPERATING EXPENSES	10,384	9,793
OPERATING INCOME	1,183	1,110
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(3)	56
Interest, net	13	9
Intercompany charges, net	(71)	(81)
Other, net	(8)	(5)
INCOME BEFORE INCOME TAXES	1,114	1,089
PROVISION FOR INCOME TAXES	399	380
NET INCOME	715	709
COMPREHENSIVE INCOME	719	702
Non Guarantor Subsidiaries [Member]
Condensed Financial Statements Captions [Line Items]
REVENUES	3,854	3,830
OPERATING EXPENSES:
Salaries and employee benefits	1,253	1,169
Purchased transportation	1,464	1,351
Rentals and landing fees	191	170
Depreciation and amortization	112	128
Fuel	66	72
Maintenance and repairs	73	72
Intercompany charges, net	3	28
Other	758	686
OPERATING EXPENSES	3,920	3,676
OPERATING INCOME	(66)	154
OTHER INCOME (EXPENSE):
Interest, net	2
Intercompany charges, net	(60)	(41)
Other, net	(11)	(4)
INCOME BEFORE INCOME TAXES	(135)	109
PROVISION FOR INCOME TAXES	(13)	47
NET INCOME	(122)	62
COMPREHENSIVE INCOME	$ (18)	$ 81