Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Feb. 28, 2019	May 31, 2018	Feb. 28, 2018	May 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 2,872	$ 3,265	$ 2,789	$ 3,969
Receivables, less allowances	9,037	8,481
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	1,591	1,595
Total current assets	13,500	13,341
PROPERTY AND EQUIPMENT, AT COST	58,164	55,121
Less accumulated depreciation and amortization	28,396	26,967
Net property and equipment	29,768	28,154
GOODWILL	6,916	6,973
OTHER ASSETS	4,280	3,862
ASSETS	54,464	52,330
CURRENT LIABILITIES
Short-term borrowings	225
Current portion of long-term debt	973	1,342
Accrued salaries and employee benefits	1,659	2,177
Accounts payable	3,156	2,977
Accrued expenses	3,243	3,131
Total current liabilities	9,256	9,627
LONG-TERM DEBT, LESS CURRENT PORTION	17,218	15,243
OTHER LONG-TERM LIABILITIES
Deferred income taxes	3,211	2,867
Other liabilities	4,885	5,177
Total other long-term liabilities	8,096	8,044
COMMON STOCKHOLDERS’ INVESTMENT	19,894	19,416	18,894
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT	54,464	52,330
Consolidation Eliminations [Member]
CURRENT ASSETS
Cash and cash equivalents	(41)	(15)	(37)	(47)
Receivables, less allowances	(82)	(78)
Total current assets	(123)	(93)
INTERCOMPANY RECEIVABLE	(2,818)	(2,411)
INVESTMENT IN SUBSIDIARIES	(40,707)	(37,452)
OTHER ASSETS	(138)	104
ASSETS	(43,786)	(39,852)
CURRENT LIABILITIES
Accounts payable	(118)	(90)
Accrued expenses	(5)	(3)
Total current liabilities	(123)	(93)
INTERCOMPANY PAYABLE	(2,818)	(2,411)
OTHER LONG-TERM LIABILITIES
Deferred income taxes	(138)	104
Total other long-term liabilities	(138)	104
COMMON STOCKHOLDERS’ INVESTMENT	(40,707)	(37,452)
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT	(43,786)	(39,852)
Parent Company [Member] | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents	1,413	1,485	957	1,884
Receivables, less allowances	45	3
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	313	425
Total current assets	1,771	1,913
PROPERTY AND EQUIPMENT, AT COST	23	21
Less accumulated depreciation and amortization	17	17
Net property and equipment	6	4
INTERCOMPANY RECEIVABLE	661	1,487
INVESTMENT IN SUBSIDIARIES	35,803	33,370
OTHER ASSETS	891	75
ASSETS	39,132	36,849
CURRENT LIABILITIES
Short-term borrowings	225
Current portion of long-term debt	966	1,332
Accrued salaries and employee benefits	52	65
Accounts payable	188	16
Accrued expenses	510	460
Total current liabilities	1,941	1,873
LONG-TERM DEBT, LESS CURRENT PORTION	16,921	14,942
OTHER LONG-TERM LIABILITIES
Other liabilities	376	619
Total other long-term liabilities	376	619
COMMON STOCKHOLDERS’ INVESTMENT	19,894	19,415
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT	39,132	36,849
Guarantor Subsidiaries [Member[ | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents	204	257	375	325
Receivables, less allowances	5,474	4,970
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	979	878
Total current assets	6,657	6,105
PROPERTY AND EQUIPMENT, AT COST	53,997	51,232
Less accumulated depreciation and amortization	26,384	25,111
Net property and equipment	27,613	26,121
INTERCOMPANY RECEIVABLE	2,157	924
GOODWILL	1,598	1,709
INVESTMENT IN SUBSIDIARIES	4,904	4,082
OTHER ASSETS	1,670	1,854
ASSETS	44,599	40,795
CURRENT LIABILITIES
Current portion of long-term debt	2	1
Accrued salaries and employee benefits	1,149	1,506
Accounts payable	1,430	1,332
Accrued expenses	1,848	1,778
Total current liabilities	4,429	4,617
LONG-TERM DEBT, LESS CURRENT PORTION	287	288
OTHER LONG-TERM LIABILITIES
Deferred income taxes	3,252	2,626
Other liabilities	3,502	3,432
Total other long-term liabilities	6,754	6,058
COMMON STOCKHOLDERS’ INVESTMENT	33,129	29,832
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT	44,599	40,795
Non Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents	1,296	1,538	$ 1,494	$ 1,807
Receivables, less allowances	3,600	3,586
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	299	292
Total current assets	5,195	5,416
PROPERTY AND EQUIPMENT, AT COST	4,144	3,868
Less accumulated depreciation and amortization	1,995	1,839
Net property and equipment	2,149	2,029
GOODWILL	5,318	5,264
OTHER ASSETS	1,857	1,829
ASSETS	14,519	14,538
CURRENT LIABILITIES
Current portion of long-term debt	5	9
Accrued salaries and employee benefits	458	606
Accounts payable	1,656	1,719
Accrued expenses	890	896
Total current liabilities	3,009	3,230
LONG-TERM DEBT, LESS CURRENT PORTION	10	13
INTERCOMPANY PAYABLE	2,818	2,411
OTHER LONG-TERM LIABILITIES
Deferred income taxes	97	137
Other liabilities	1,007	1,126
Total other long-term liabilities	1,104	1,263
COMMON STOCKHOLDERS’ INVESTMENT	7,578	7,621
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT	$ 14,519	$ 14,538