Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions		May 31, 2019	May 31, 2018	May 31, 2017	May 31, 2016
CURRENT ASSETS
Cash and cash equivalents		$ 2,319	$ 3,265	$ 3,969	$ 3,534
Receivables, less allowances		9,116	8,481
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		1,651	1,595
Total current assets		13,086	13,341
PROPERTY AND EQUIPMENT, AT COST		59,511	55,121
Less accumulated depreciation and amortization		29,082	26,967
Net property and equipment		30,429	28,154
GOODWILL		6,884	6,973	7,154
OTHER ASSETS		4,004	3,862
ASSETS	[1]	54,403	52,330	48,552
CURRENT LIABILITIES
Current portion of long-term debt		964	1,342
Accrued salaries and employee benefits		1,741	2,177
Accounts payable		3,030	2,977
Accrued expenses		3,278	3,131
Total current liabilities		9,013	9,627
LONG-TERM DEBT, LESS CURRENT PORTION		16,617	15,243
OTHER LONG-TERM LIABILITIES
Deferred income taxes		2,821	2,867
Other liabilities		8,195	5,177
Total other long-term liabilities		11,016	8,044
COMMON STOCKHOLDERS’ INVESTMENT		17,757	19,416	16,073	13,784
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT		54,403	52,330
Consolidation Eliminations [Member]
CURRENT ASSETS
Cash and cash equivalents		(46)	(15)	(47)	(43)
Receivables, less allowances		(227)	(78)
Total current assets		(273)	(93)
INTERCOMPANY RECEIVABLE		(2,472)	(2,411)
INVESTMENT IN SUBSIDIARIES		(39,174)	(37,452)
OTHER ASSETS		(591)	104
ASSETS		(42,510)	(39,852)
CURRENT LIABILITIES
Accounts payable		(263)	(90)
Accrued expenses		(10)	(3)
Total current liabilities		(273)	(93)
INTERCOMPANY PAYABLE		(2,472)	(2,411)
OTHER LONG-TERM LIABILITIES
Deferred income taxes		(591)	104
Total other long-term liabilities		(591)	104
COMMON STOCKHOLDERS’ INVESTMENT		(39,174)	(37,452)
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT		(42,510)	(39,852)
Parent Company [Member] | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents		826	1,485	1,884	1,974
Receivables, less allowances		56	3
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		366	425
Total current assets		1,248	1,913
PROPERTY AND EQUIPMENT, AT COST		25	21
Less accumulated depreciation and amortization		17	17
Net property and equipment		8	4
INTERCOMPANY RECEIVABLE		2,877	1,487
INVESTMENT IN SUBSIDIARIES		33,725	33,370
OTHER ASSETS		995	75
ASSETS		38,853	36,849
CURRENT LIABILITIES
Current portion of long-term debt		959	1,332
Accrued salaries and employee benefits		143	65
Accounts payable		16	16
Accrued expenses		521	460
Total current liabilities		1,639	1,873
LONG-TERM DEBT, LESS CURRENT PORTION		16,322	14,942
OTHER LONG-TERM LIABILITIES
Other liabilities		3,135	619
Total other long-term liabilities		3,135	619
COMMON STOCKHOLDERS’ INVESTMENT		17,757	19,415
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT		38,853	36,849
Guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents		158	257	325	326
Receivables, less allowances		5,603	4,970
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		953	878
Total current assets		6,714	6,105
PROPERTY AND EQUIPMENT, AT COST		55,341	51,232
Less accumulated depreciation and amortization		27,066	25,111
Net property and equipment		28,275	26,121
INTERCOMPANY RECEIVABLE		(405)	924
GOODWILL		1,589	1,709
INVESTMENT IN SUBSIDIARIES		5,449	4,082
OTHER ASSETS		1,811	1,854
ASSETS		43,433	40,795
CURRENT LIABILITIES
Current portion of long-term debt		2	1
Accrued salaries and employee benefits		1,100	1,506
Accounts payable		1,469	1,332
Accrued expenses		1,853	1,778
Total current liabilities		4,424	4,617
LONG-TERM DEBT, LESS CURRENT PORTION		287	288
OTHER LONG-TERM LIABILITIES
Deferred income taxes		2,832	2,626
Other liabilities		3,965	3,432
Total other long-term liabilities		6,797	6,058
COMMON STOCKHOLDERS’ INVESTMENT		31,925	29,832
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT		43,433	40,795
Non-guarantor Subsidiaries [Member] | Reportable Legal Entities [Member]
CURRENT ASSETS
Cash and cash equivalents		1,381	1,538	$ 1,807	$ 1,277
Receivables, less allowances		3,684	3,586
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		332	292
Total current assets		5,397	5,416
PROPERTY AND EQUIPMENT, AT COST		4,145	3,868
Less accumulated depreciation and amortization		1,999	1,839
Net property and equipment		2,146	2,029
GOODWILL		5,295	5,264
OTHER ASSETS		1,789	1,829
ASSETS		14,627	14,538
CURRENT LIABILITIES
Current portion of long-term debt		3	9
Accrued salaries and employee benefits		498	606
Accounts payable		1,808	1,719
Accrued expenses		914	896
Total current liabilities		3,223	3,230
LONG-TERM DEBT, LESS CURRENT PORTION		8	13
INTERCOMPANY PAYABLE		2,472	2,411
OTHER LONG-TERM LIABILITIES
Deferred income taxes		580	137
Other liabilities		1,095	1,126
Total other long-term liabilities		1,675	1,263
COMMON STOCKHOLDERS’ INVESTMENT		7,249	7,621
LIABILITIES AND COMMON STOCKHOLDERS' INVESTMENT		$ 14,627	$ 14,538

[1] Segment assets include intercompany receivables.