Condensed Consolidating Financial Statements (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Nov. 30, 2014	Nov. 30, 2013	Nov. 30, 2014	Nov. 30, 2013
Condensed Financial Statements Captions [Line Items]
REVENUES	$ 11,939	$ 11,403	$ 23,623	$ 22,427
OPERATING EXPENSES:
Salaries and employee benefits	4,304	4,148	8,493	8,225
Purchased transportation	2,185	2,040	4,239	3,919
Rentals and landing fees	663	648	1,323	1,288
Depreciation and amortization	651	647	1,302	1,286
Fuel	1,052	1,136	2,172	2,240
Maintenance and repairs	543	479	1,099	959
Other	1,528	1,478	2,995	2,888
OPERATING EXPENSES	10,926	10,576	21,623	20,805
OPERATING INCOME	1,013	827	2,000	1,622
OTHER INCOME (EXPENSE):
Interest, net	(47)	(30)	(95)	(57)
Other, net	5	(5)	3	(7)
INCOME BEFORE INCOME TAXES	971	792	1,908	1,558
PROVISION FOR INCOME TAXES	355	292	686	569
NET INCOME	616	500	1,222	989
COMPREHENSIVE INCOME	523	588	1,129	1,040
Parent Company Member
OPERATING EXPENSES:
Salaries and employee benefits	23	24	53	55
Rentals and landing fees	2	2	3	3
Depreciation and amortization	1	1	1	1
Intercompany charges, net	(48)	(50)	(143)	(111)
Other	22	23	86	52
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	616	500	1,222	989
Interest, net	(53)	(35)	(106)	(69)
Intercompany charges, net	54	36	108	71
Other, net	(1)	(1)	(2)	(2)
INCOME BEFORE INCOME TAXES	616	500	1,222	989
NET INCOME	616	500	1,222	989
COMPREHENSIVE INCOME	642	540	1,276	1,068
Guarantor Subsidiaries Member
Condensed Financial Statements Captions [Line Items]
REVENUES	9,926	9,447	19,695	18,675
OPERATING EXPENSES:
Salaries and employee benefits	3,729	3,568	7,335	7,082
Purchased transportation	1,468	1,340	2,854	2,582
Rentals and landing fees	577	563	1,149	1,121
Depreciation and amortization	595	595	1,190	1,184
Fuel	1,028	1,111	2,123	2,192
Maintenance and repairs	507	447	1,029	896
Intercompany charges, net	(50)	(24)	(48)	(30)
Other	1,239	1,192	2,404	2,325
OPERATING EXPENSES	9,093	8,792	18,036	17,352
OPERATING INCOME	833	655	1,659	1,323
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	103	133	201	243
Interest, net	5	5	9	10
Intercompany charges, net	(59)	(41)	(118)	(82)
Other, net	2	(3)	(1)	(5)
INCOME BEFORE INCOME TAXES	884	749	1,750	1,489
PROVISION FOR INCOME TAXES	319	251	588	483
NET INCOME	565	498	1,162	1,006
COMPREHENSIVE INCOME	543	506	1,137	1,013
Non Guarantor Subsidiaries Member
Condensed Financial Statements Captions [Line Items]
REVENUES	2,108	2,042	4,112	3,920
OPERATING EXPENSES:
Salaries and employee benefits	552	556	1,105	1,088
Purchased transportation	764	739	1,475	1,412
Rentals and landing fees	86	85	174	167
Depreciation and amortization	55	51	111	101
Fuel	24	25	49	48
Maintenance and repairs	36	32	70	63
Intercompany charges, net	98	74	191	141
Other	313	308	596	601
OPERATING EXPENSES	1,928	1,870	3,771	3,621
OPERATING INCOME	180	172	341	299
OTHER INCOME (EXPENSE):
Interest, net	1		2	2
Intercompany charges, net	5	5	10	11
Other, net	4	(1)	6
INCOME BEFORE INCOME TAXES	190	176	359	312
PROVISION FOR INCOME TAXES	36	41	98	86
NET INCOME	154	135	261	226
COMPREHENSIVE INCOME	57	175	139	191
Consolidation Eliminations Member
Condensed Financial Statements Captions [Line Items]
REVENUES	(95)	(86)	(184)	(168)
OPERATING EXPENSES:
Purchased transportation	(47)	(39)	(90)	(75)
Rentals and landing fees	(2)	(2)	(3)	(3)
Other	(46)	(45)	(91)	(90)
OPERATING EXPENSES	(95)	(86)	(184)	(168)
OTHER INCOME (EXPENSE):
Equity in earnings of subsidiaries	(719)	(633)	(1,423)	(1,232)
INCOME BEFORE INCOME TAXES	(719)	(633)	(1,423)	(1,232)
NET INCOME	(719)	(633)	(1,423)	(1,232)
COMPREHENSIVE INCOME	$ (719)	$ (633)	$ (1,423)	$ (1,232)