Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions		May 31, 2017	May 31, 2016	May 31, 2015	May 31, 2014
CURRENT ASSETS
Cash and cash equivalents		$ 3,969	$ 3,534	$ 3,763	$ 2,908
Receivables, less allowances		7,599	7,252
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		1,060	1,203
Total current assets		12,628	11,989
PROPERTY AND EQUIPMENT, AT COST		50,626	47,018
Less accumulated depreciation and amortization		24,645	22,734
Net property and equipment		25,981	24,284
GOODWILL		7,154	6,747	3,810
OTHER ASSETS		2,789	2,939
ASSETS	[1]	48,552	45,959	36,469
CURRENT LIABILITIES
Current portion of long-term debt		22	29
Accrued salaries and employee benefits		1,914	1,972
Accounts payable		2,752	2,944
Accrued expenses		3,230	3,063
Total current liabilities		7,918	8,008
LONG-TERM DEBT, LESS CURRENT PORTION		14,909	13,733
OTHER LONG-TERM LIABILITIES
Deferred income taxes		2,485	1,567
Other liabilities		7,167	8,867
Total other long-term liabilities		9,652	10,434
STOCKHOLDERS’ INVESTMENT		16,073	13,784	14,993	15,277
LIABILITIES AND STOCKHOLDERS' INVESTMENT		48,552	45,959
Consolidation Eliminations [Member]
CURRENT ASSETS
Cash and cash equivalents		(47)	(43)	(78)	(150)
Receivables, less allowances		(61)	(41)
Total current assets		(108)	(84)
INTERCOMPANY RECEIVABLE		(4,128)	(3,721)
INVESTMENT IN SUBSIDIARIES		(30,348)	(28,463)
OTHER ASSETS		(3,225)	(3,238)
ASSETS		(37,809)	(35,506)
CURRENT LIABILITIES
Accounts payable		(108)	(84)
Total current liabilities		(108)	(84)
INTERCOMPANY PAYABLE		(4,128)	(3,721)
OTHER LONG-TERM LIABILITIES
Deferred income taxes		(3,225)	(3,238)
Total other long-term liabilities		(3,225)	(3,238)
STOCKHOLDERS’ INVESTMENT		(30,348)	(28,463)
LIABILITIES AND STOCKHOLDERS' INVESTMENT		(37,809)	(35,506)
Parent Company [Member]
CURRENT ASSETS
Cash and cash equivalents		1,884	1,974	2,383	1,756
Receivables, less allowances		3	1
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		25	233
Total current assets		1,912	2,208
PROPERTY AND EQUIPMENT, AT COST		22	22
Less accumulated depreciation and amortization		18	17
Net property and equipment		4	5
INTERCOMPANY RECEIVABLE		1,521	2,437
INVESTMENT IN SUBSIDIARIES		27,712	24,766
OTHER ASSETS		3,494	3,359
ASSETS		34,643	32,775
CURRENT LIABILITIES
Accrued salaries and employee benefits		72	54
Accounts payable		10	8
Accrued expenses		991	883
Total current liabilities		1,073	945
LONG-TERM DEBT, LESS CURRENT PORTION		14,641	13,451
OTHER LONG-TERM LIABILITIES
Other liabilities		2,856	4,595
Total other long-term liabilities		2,856	4,595
STOCKHOLDERS’ INVESTMENT		16,073	13,784
LIABILITIES AND STOCKHOLDERS' INVESTMENT		34,643	32,775
Guarantor Subsidiaries [Member[
CURRENT ASSETS
Cash and cash equivalents		325	326	487	441
Receivables, less allowances		4,729	4,461
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		787	724
Total current assets		5,841	5,511
PROPERTY AND EQUIPMENT, AT COST		47,201	43,760
Less accumulated depreciation and amortization		23,211	21,566
Net property and equipment		23,990	22,194
INTERCOMPANY RECEIVABLE		2,607	1,284
GOODWILL		1,571	1,571
INVESTMENT IN SUBSIDIARIES		2,636	3,697
OTHER ASSETS		1,271	967
ASSETS		37,916	35,224
CURRENT LIABILITIES
Current portion of long-term debt		9	13
Accrued salaries and employee benefits		1,335	1,377
Accounts payable		1,411	1,501
Accrued expenses		1,522	1,411
Total current liabilities		4,277	4,302
LONG-TERM DEBT, LESS CURRENT PORTION		244	245
OTHER LONG-TERM LIABILITIES
Deferred income taxes		5,472	4,436
Other liabilities		3,448	3,375
Total other long-term liabilities		8,920	7,811
STOCKHOLDERS’ INVESTMENT		24,475	22,866
LIABILITIES AND STOCKHOLDERS' INVESTMENT		37,916	35,224
Non Guarantor Subsidiaries [Member]
CURRENT ASSETS
Cash and cash equivalents		1,807	1,277	$ 971	$ 861
Receivables, less allowances		2,928	2,831
Spare parts, supplies, fuel, prepaid expenses and other, less allowances		248	246
Total current assets		4,983	4,354
PROPERTY AND EQUIPMENT, AT COST		3,403	3,236
Less accumulated depreciation and amortization		1,416	1,151
Net property and equipment		1,987	2,085
GOODWILL		5,583	5,176
OTHER ASSETS		1,249	1,851
ASSETS		13,802	13,466
CURRENT LIABILITIES
Current portion of long-term debt		13	16
Accrued salaries and employee benefits		507	541
Accounts payable		1,439	1,519
Accrued expenses		717	769
Total current liabilities		2,676	2,845
LONG-TERM DEBT, LESS CURRENT PORTION		24	37
INTERCOMPANY PAYABLE		4,128	3,721
OTHER LONG-TERM LIABILITIES
Deferred income taxes		238	369
Other liabilities		863	897
Total other long-term liabilities		1,101	1,266
STOCKHOLDERS’ INVESTMENT		5,873	5,597
LIABILITIES AND STOCKHOLDERS' INVESTMENT		$ 13,802	$ 13,466

[1] Segment assets include intercompany receivables.