Condensed Consolidating Financial Statements (Details 2) - USD ($) $ in Millions	May 31, 2016	May 31, 2015	May 31, 2014	May 31, 2013
CURRENT ASSETS
Cash and cash equivalents	$ 3,534	$ 3,763	$ 2,908	$ 4,917
Receivables, less allowances	7,252	5,719
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	1,203	853
Total current assets	11,989	10,335
PROPERTY AND EQUIPMENT, AT COST	47,018	42,864
Less accumulated depreciation and amortization	22,734	21,989
Net property and equipment	24,284	20,875
GOODWILL	6,747	3,810	2,790
Other assets	3,044	1,511
ASSETS	46,064	36,531	33,070
CURRENT LIABILITIES
Current portion of long-term debt	29	19
Accrued salaries and employee benefits	1,972	1,436
Accounts payable	2,944	2,066
Accrued expenses	3,063	2,435
Total current liabilities	8,008	5,956
LONG-TERM DEBT, LESS CURRENT PORTION	13,838	7,249
OTHER LONG-TERM LIABILITIES
Deferred income taxes	1,567	1,210
Other liabilities	8,867	7,123
Total other long-term liabilities	10,434	8,333
STOCKHOLDERS' INVESTMENT	13,784	14,993	15,277	17,398
LIABILITIES AND STOCKHOLDERS' INVESTMENT	46,064	36,531
Parent Company [Member]
CURRENT ASSETS
Cash and cash equivalents	1,974	2,383	1,756	3,892
Receivables, less allowances	1	3
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	233	41
Total current assets	2,208	2,427
PROPERTY AND EQUIPMENT, AT COST	22	29
Less accumulated depreciation and amortization	17	23
Net property and equipment	5	6
INTERCOMPANY RECEIVABLE	2,437
INVESTMENT IN SUBSIDIARIES	24,766	23,173
Other assets	3,461	2,770
ASSETS	32,877	28,376
CURRENT LIABILITIES
Accrued salaries and employee benefits	54	34
Accounts payable	8	5
Accrued expenses	883	604
Total current liabilities	945	643
LONG-TERM DEBT, LESS CURRENT PORTION	13,553	6,978
INTERCOMPANY PAYABLE		2,267
OTHER LONG-TERM LIABILITIES
Other liabilities	4,595	3,495
Total other long-term liabilities	4,595	3,495
STOCKHOLDERS' INVESTMENT	13,784	14,993
LIABILITIES AND STOCKHOLDERS' INVESTMENT	32,877	28,376
Guarantor Subsidiaries [Member]
CURRENT ASSETS
Cash and cash equivalents	326	487	441	405
Receivables, less allowances	4,461	4,383
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	724	689
Total current assets	5,511	5,559
PROPERTY AND EQUIPMENT, AT COST	43,760	40,364
Less accumulated depreciation and amortization	21,566	20,685
Net property and equipment	22,194	19,679
INTERCOMPANY RECEIVABLE	1,284	713
GOODWILL	1,571	1,552
INVESTMENT IN SUBSIDIARIES	3,697	3,800
Other assets	970	898
ASSETS	35,227	32,201
CURRENT LIABILITIES
Current portion of long-term debt	13	7
Accrued salaries and employee benefits	1,377	1,208
Accounts payable	1,501	1,433
Accrued expenses	1,411	1,557
Total current liabilities	4,302	4,205
LONG-TERM DEBT, LESS CURRENT PORTION	248	248
OTHER LONG-TERM LIABILITIES
Deferred income taxes	4,436	3,662
Other liabilities	3,375	3,367
Total other long-term liabilities	7,811	7,029
STOCKHOLDERS' INVESTMENT	22,866	20,719
LIABILITIES AND STOCKHOLDERS' INVESTMENT	35,227	32,201
Non Guarantor Subsidiaries [Member]
CURRENT ASSETS
Cash and cash equivalents	1,277	971	861	717
Receivables, less allowances	2,831	1,385
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	246	123
Total current assets	4,354	2,479
PROPERTY AND EQUIPMENT, AT COST	3,236	2,471
Less accumulated depreciation and amortization	1,151	1,281
Net property and equipment	2,085	1,190
INTERCOMPANY RECEIVABLE		1,554
GOODWILL	5,176	2,258
Other assets	1,851	480
ASSETS	13,466	7,961
CURRENT LIABILITIES
Current portion of long-term debt	16	12
Accrued salaries and employee benefits	541	194
Accounts payable	1,519	758
Accrued expenses	769	274
Total current liabilities	2,845	1,238
LONG-TERM DEBT, LESS CURRENT PORTION	37	23
INTERCOMPANY PAYABLE	3,721
OTHER LONG-TERM LIABILITIES
Deferred income taxes	369	185
Other liabilities	897	261
Total other long-term liabilities	1,266	446
STOCKHOLDERS' INVESTMENT	5,597	6,254
LIABILITIES AND STOCKHOLDERS' INVESTMENT	13,466	7,961
Consolidation Eliminations [Member]
CURRENT ASSETS
Cash and cash equivalents	(43)	(78)	$ (150)	$ (97)
Receivables, less allowances	(41)	(52)
Total current assets	(84)	(130)
INTERCOMPANY RECEIVABLE	(3,721)	(2,267)
INVESTMENT IN SUBSIDIARIES	(28,463)	(26,973)
Other assets	(3,238)	(2,637)
ASSETS	(35,506)	(32,007)
CURRENT LIABILITIES
Accounts payable	(84)	(130)
Total current liabilities	(84)	(130)
INTERCOMPANY PAYABLE	(3,721)	(2,267)
OTHER LONG-TERM LIABILITIES
Deferred income taxes	(3,238)	(2,637)
Total other long-term liabilities	(3,238)	(2,637)
STOCKHOLDERS' INVESTMENT	(28,463)	(26,973)
LIABILITIES AND STOCKHOLDERS' INVESTMENT	$ (35,506)	$ (32,007)