Condensed Consolidating Financial Statements (Details 3) - USD ($) $ in Millions	3 Months Ended
	Aug. 31, 2016	Aug. 31, 2015
Condensed Financial Statements Captions [Line Items]
REVENUES	$ 14,663	$ 12,279
OPERATING EXPENSES:
Salaries and employee benefits	5,311	4,525
Purchased transportation	3,240	2,344
Rentals and landing fees	790	695
Depreciation and amortization	739	648
Fuel	650	712
Maintenance and repairs	598	548
Other	2,071	1,663
OPERATING EXPENSES	13,399	11,135
OPERATING INCOME	1,264	1,144
OTHER INCOME (EXPENSE):
Interest, net	(113)	(63)
Other, net	(9)	3
INCOME BEFORE INCOME TAXES	1,142	1,084
PROVISION FOR INCOME TAXES	427	392
NET INCOME	715	692
COMPREHENSIVE INCOME	708	530
Parent Company Member
OPERATING EXPENSES:
Salaries and employee benefits	36	34
Rentals and landing fees	1	1
Intercompany charges, net	(90)	(69)
Other	53	34
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	715	692
Interest, net	(122)	(75)
Intercompany charges, net	122	78
Other, net		(3)
INCOME BEFORE INCOME TAXES	715	692
NET INCOME	715	692
COMPREHENSIVE INCOME	696	674
Guarantor Subsidiaries Member
Condensed Financial Statements Captions [Line Items]
REVENUES	10,903	9,873
OPERATING EXPENSES:
Salaries and employee benefits	4,106	3,813
Purchased transportation	1,917	1,434
Rentals and landing fees	620	587
Depreciation and amortization	611	583
Fuel	578	691
Maintenance and repairs	526	508
Intercompany charges, net	62	(40)
Other	1,373	1,264
OPERATING EXPENSES	9,793	8,840
OPERATING INCOME	1,110	1,033
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	56	61
Interest, net	9	8
Intercompany charges, net	(81)	(76)
Other, net	(5)	(3)
INCOME BEFORE INCOME TAXES	1,089	1,023
PROVISION FOR INCOME TAXES	380	357
NET INCOME	709	666
COMPREHENSIVE INCOME	702	651
Non Guarantor Subsidiaries Member
Condensed Financial Statements Captions [Line Items]
REVENUES	3,830	2,509
OPERATING EXPENSES:
Salaries and employee benefits	1,169	678
Purchased transportation	1,351	965
Rentals and landing fees	170	108
Depreciation and amortization	128	65
Fuel	72	21
Maintenance and repairs	72	40
Intercompany charges, net	28	109
Other	686	412
OPERATING EXPENSES	3,676	2,398
OPERATING INCOME	154	111
OTHER INCOME (EXPENSE):
Interest, net		4
Intercompany charges, net	(41)	(2)
Other, net	(4)	9
INCOME BEFORE INCOME TAXES	109	122
PROVISION FOR INCOME TAXES	47	35
NET INCOME	62	87
COMPREHENSIVE INCOME	81	(42)
Consolidation Eliminations Member
Condensed Financial Statements Captions [Line Items]
REVENUES	(70)	(103)
OPERATING EXPENSES:
Purchased transportation	(28)	(55)
Rentals and landing fees	(1)	(1)
Other	(41)	(47)
OPERATING EXPENSES	(70)	(103)
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(771)	(753)
INCOME BEFORE INCOME TAXES	(771)	(753)
NET INCOME	(771)	(753)
COMPREHENSIVE INCOME	$ (771)	$ (753)