Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Aug. 31, 2017	May 31, 2017	Aug. 31, 2016	May 31, 2016
CURRENT ASSETS
Cash and cash equivalents	$ 3,503	$ 3,969	$ 2,989	$ 3,534
Receivables, less allowances	8,006	7,599
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	1,213	1,060
Total current assets	12,722	12,628
PROPERTY AND EQUIPMENT, AT COST	51,540	50,626
Less accumulated depreciation and amortization	25,305	24,645
Net property and equipment	26,235	25,981
GOODWILL	7,382	7,154
OTHER ASSETS	3,011	2,789
ASSETS	49,350	48,552
CURRENT LIABILITIES
Current portion of long-term debt	19	22
Accrued salaries and employee benefits	1,656	1,914
Accounts payable	2,938	2,752
Accrued expenses	3,177	3,230
Total current liabilities	7,790	7,918
LONG-TERM DEBT, LESS CURRENT PORTION	15,137	14,909
OTHER LONG-TERM LIABILITIES
Deferred income taxes	2,730	2,485
Other liabilities	7,075	7,167
Total other long-term liabilities	9,805	9,652
STOCKHOLDERS’ INVESTMENT	16,618	16,073
LIABILITIES AND STOCKHOLDERS' INVESTMENT	49,350	48,552
Consolidation Eliminations [Member]
CURRENT ASSETS
Cash and cash equivalents	(40)	(47)	(37)	(43)
Receivables, less allowances	(89)	(61)
Total current assets	(129)	(108)
INTERCOMPANY RECEIVABLE	(4,322)	(4,128)
INVESTMENT IN SUBSIDIARIES	(31,105)	(30,348)
OTHER ASSETS	(3,192)	(3,225)
ASSETS	(38,748)	(37,809)
CURRENT LIABILITIES
Accounts payable	(129)	(108)
Total current liabilities	(129)	(108)
INTERCOMPANY PAYABLE	(4,322)	(4,128)
OTHER LONG-TERM LIABILITIES
Deferred income taxes	(3,192)	(3,225)
Total other long-term liabilities	(3,192)	(3,225)
STOCKHOLDERS’ INVESTMENT	(31,105)	(30,348)
LIABILITIES AND STOCKHOLDERS' INVESTMENT	(38,748)	(37,809)
Parent Company [Member]
CURRENT ASSETS
Cash and cash equivalents	1,681	1,884	1,308	1,974
Receivables, less allowances	2	3
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	41	25
Total current assets	1,724	1,912
PROPERTY AND EQUIPMENT, AT COST	22	22
Less accumulated depreciation and amortization	18	18
Net property and equipment	4	4
INTERCOMPANY RECEIVABLE	1,578	1,521
INVESTMENT IN SUBSIDIARIES	28,433	27,712
OTHER ASSETS	3,492	3,494
ASSETS	35,231	34,643
CURRENT LIABILITIES
Accrued salaries and employee benefits	50	72
Accounts payable	149	10
Accrued expenses	891	991
Total current liabilities	1,090	1,073
LONG-TERM DEBT, LESS CURRENT PORTION	14,872	14,641
OTHER LONG-TERM LIABILITIES
Other liabilities	2,651	2,856
Total other long-term liabilities	2,651	2,856
STOCKHOLDERS’ INVESTMENT	16,618	16,073
LIABILITIES AND STOCKHOLDERS' INVESTMENT	35,231	34,643
Guarantor Subsidiaries [Member]
CURRENT ASSETS
Cash and cash equivalents	343	325	327	326
Receivables, less allowances	4,852	4,729
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	931	787
Total current assets	6,126	5,841
PROPERTY AND EQUIPMENT, AT COST	47,939	47,201
Less accumulated depreciation and amortization	23,761	23,211
Net property and equipment	24,178	23,990
INTERCOMPANY RECEIVABLE	2,744	2,607
GOODWILL	1,570	1,571
INVESTMENT IN SUBSIDIARIES	2,672	2,636
OTHER ASSETS	1,403	1,271
ASSETS	38,693	37,916
CURRENT LIABILITIES
Current portion of long-term debt	6	9
Accrued salaries and employee benefits	1,116	1,335
Accounts payable	1,429	1,411
Accrued expenses	1,520	1,522
Total current liabilities	4,071	4,277
LONG-TERM DEBT, LESS CURRENT PORTION	244	244
OTHER LONG-TERM LIABILITIES
Deferred income taxes	5,753	5,472
Other liabilities	3,526	3,448
Total other long-term liabilities	9,279	8,920
STOCKHOLDERS’ INVESTMENT	25,099	24,475
LIABILITIES AND STOCKHOLDERS' INVESTMENT	38,693	37,916
Non Guarantor Subsidiaries [Member]
CURRENT ASSETS
Cash and cash equivalents	1,519	1,807	$ 1,391	$ 1,277
Receivables, less allowances	3,241	2,928
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	241	248
Total current assets	5,001	4,983
PROPERTY AND EQUIPMENT, AT COST	3,579	3,403
Less accumulated depreciation and amortization	1,526	1,416
Net property and equipment	2,053	1,987
GOODWILL	5,812	5,583
OTHER ASSETS	1,308	1,249
ASSETS	14,174	13,802
CURRENT LIABILITIES
Current portion of long-term debt	13	13
Accrued salaries and employee benefits	490	507
Accounts payable	1,489	1,439
Accrued expenses	766	717
Total current liabilities	2,758	2,676
LONG-TERM DEBT, LESS CURRENT PORTION	21	24
INTERCOMPANY PAYABLE	4,322	4,128
OTHER LONG-TERM LIABILITIES
Deferred income taxes	169	238
Other liabilities	898	863
Total other long-term liabilities	1,067	1,101
STOCKHOLDERS’ INVESTMENT	6,006	5,873
LIABILITIES AND STOCKHOLDERS' INVESTMENT	$ 14,174	$ 13,802