Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Nov. 30, 2018	May 31, 2018	Nov. 30, 2017	May 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 2,123	$ 3,265	$ 2,768	$ 3,969
Receivables, less allowances	9,573	8,481
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	1,742	1,595
Total current assets	13,438	13,341
PROPERTY AND EQUIPMENT, AT COST	57,501	55,121
Less accumulated depreciation and amortization	28,114	26,967
Net property and equipment	29,387	28,154
GOODWILL	6,908	6,973
OTHER ASSETS	3,556	3,862
ASSETS	53,289	52,330
CURRENT LIABILITIES
Short-term borrowings	250
Current portion of long-term debt	642	1,342
Accrued salaries and employee benefits	1,850	2,177
Accounts payable	3,400	2,977
Accrued expenses	3,354	3,131
Total current liabilities	9,496	9,627
LONG-TERM DEBT, LESS CURRENT PORTION	16,399	15,243
OTHER LONG-TERM LIABILITIES
Deferred income taxes	3,253	2,867
Other liabilities	4,847	5,177
Total other long-term liabilities	8,100	8,044
STOCKHOLDERS’ INVESTMENT	19,294	19,416
LIABILITIES AND STOCKHOLDERS' INVESTMENT	53,289	52,330
Consolidation Eliminations [Member]
CURRENT ASSETS
Cash and cash equivalents	(62)	(15)	(28)	(47)
Receivables, less allowances	(183)	(78)
Total current assets	(245)	(93)
INTERCOMPANY RECEIVABLE	(2,473)	(2,411)
INVESTMENT IN SUBSIDIARIES	(39,848)	(37,452)
OTHER ASSETS		104
ASSETS	(42,566)	(39,852)
CURRENT LIABILITIES
Accounts payable	(204)	(90)
Accrued expenses	(40)	(3)
Total current liabilities	(244)	(93)
INTERCOMPANY PAYABLE	(2,473)	(2,411)
OTHER LONG-TERM LIABILITIES
Deferred income taxes		104
Other liabilities	(1)
Total other long-term liabilities	(1)	104
STOCKHOLDERS’ INVESTMENT	(39,848)	(37,452)
LIABILITIES AND STOCKHOLDERS' INVESTMENT	(42,566)	(39,852)
Parent Company [Member] | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents	688	1,485	959	1,884
Receivables, less allowances	40	3
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	491	425
Total current assets	1,219	1,913
PROPERTY AND EQUIPMENT, AT COST	23	21
Less accumulated depreciation and amortization	17	17
Net property and equipment	6	4
INTERCOMPANY RECEIVABLE	845	1,487
INVESTMENT IN SUBSIDIARIES	34,957	33,370
OTHER ASSETS	270	75
ASSETS	37,297	36,849
CURRENT LIABILITIES
Short-term borrowings	250
Current portion of long-term debt	567	1,332
Accrued salaries and employee benefits	49	65
Accounts payable	197	16
Accrued expenses	544	460
Total current liabilities	1,607	1,873
LONG-TERM DEBT, LESS CURRENT PORTION	16,101	14,942
OTHER LONG-TERM LIABILITIES
Deferred income taxes	92
Other liabilities	203	619
Total other long-term liabilities	295	619
STOCKHOLDERS’ INVESTMENT	19,294	19,415
LIABILITIES AND STOCKHOLDERS' INVESTMENT	37,297	36,849
Guarantor Subsidiaries [Member[ | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents	187	257	342	325
Receivables, less allowances	5,848	4,970
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	956	878
Total current assets	6,991	6,105
PROPERTY AND EQUIPMENT, AT COST	53,441	51,232
Less accumulated depreciation and amortization	26,166	25,111
Net property and equipment	27,275	26,121
INTERCOMPANY RECEIVABLE	1,628	924
GOODWILL	1,598	1,709
INVESTMENT IN SUBSIDIARIES	4,891	4,082
OTHER ASSETS	1,569	1,854
ASSETS	43,952	40,795
CURRENT LIABILITIES
Current portion of long-term debt	69	1
Accrued salaries and employee benefits	1,192	1,506
Accounts payable	1,576	1,332
Accrued expenses	1,843	1,778
Total current liabilities	4,680	4,617
LONG-TERM DEBT, LESS CURRENT PORTION	287	288
OTHER LONG-TERM LIABILITIES
Deferred income taxes	3,055	2,626
Other liabilities	3,643	3,432
Total other long-term liabilities	6,698	6,058
STOCKHOLDERS’ INVESTMENT	32,287	29,832
LIABILITIES AND STOCKHOLDERS' INVESTMENT	43,952	40,795
Non Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents	1,310	1,538	$ 1,495	$ 1,807
Receivables, less allowances	3,868	3,586
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	295	292
Total current assets	5,473	5,416
PROPERTY AND EQUIPMENT, AT COST	4,037	3,868
Less accumulated depreciation and amortization	1,931	1,839
Net property and equipment	2,106	2,029
GOODWILL	5,310	5,264
OTHER ASSETS	1,717	1,829
ASSETS	14,606	14,538
CURRENT LIABILITIES
Current portion of long-term debt	6	9
Accrued salaries and employee benefits	609	606
Accounts payable	1,831	1,719
Accrued expenses	1,007	896
Total current liabilities	3,453	3,230
LONG-TERM DEBT, LESS CURRENT PORTION	11	13
INTERCOMPANY PAYABLE	2,473	2,411
OTHER LONG-TERM LIABILITIES
Deferred income taxes	106	137
Other liabilities	1,002	1,126
Total other long-term liabilities	1,108	1,263
STOCKHOLDERS’ INVESTMENT	7,561	7,621
LIABILITIES AND STOCKHOLDERS' INVESTMENT	$ 14,606	$ 14,538