Condensed Consolidating Financial Statements (Details 2) - USD ($) $ in Millions	May. 31, 2015	May. 31, 2014	May. 31, 2013	May. 31, 2012
CURRENT ASSETS
Cash and cash equivalents	$ 3,763	$ 2,908	$ 4,917	$ 2,843
Receivables, less allowances	5,719	5,460
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	853	793
Deferred income taxes	606	522
Total current assets	10,941	9,683
PROPERTY AND EQUIPMENT, AT COST	42,864	40,691
Less accumulated depreciation and amortization	21,989	21,141
Net property and equipment	20,875	19,550
GOODWILL	3,810	2,790	2,755
Other assets	1,443	1,047
ASSETS	37,069	33,070
CURRENT LIABILITIES
Current portion of long-term debt	19	1
Accrued salaries and employee benefits	1,436	1,277
Accounts payable	2,066	1,971
Accrued expenses	2,436	2,063
Total current liabilities	5,957	5,312
LONG-TERM DEBT, LESS CURRENT PORTION	7,249	4,736
OTHER LONG-TERM LIABILITIES
Deferred income taxes	1,747	2,114
Other liabilities	7,123	5,631
Total other long-term liabilities	8,870	7,745
STOCKHOLDERS' INVESTMENT	14,993	15,277	17,398	14,727
LIABILITIES AND STOCKHOLDERS' INVESTMENT	37,069	33,070
Parent Company [Member]
CURRENT ASSETS
Cash and cash equivalents	2,383	1,756	3,892	1,906
Receivables, less allowances	3	2
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	41	59
Total current assets	2,427	1,817
PROPERTY AND EQUIPMENT, AT COST	29	28
Less accumulated depreciation and amortization	23	22
Net property and equipment	6	6
INVESTMENT IN SUBSIDIARIES	23,173	22,148
Other assets	2,752	2,088
ASSETS	28,358	26,059
CURRENT LIABILITIES
Accrued salaries and employee benefits	34	55
Accounts payable	5	2
Accrued expenses	604	405
Total current liabilities	643	462
LONG-TERM DEBT, LESS CURRENT PORTION	6,978	4,487
INTERCOMPANY PAYABLE	2,249	3,686
OTHER LONG-TERM LIABILITIES
Other liabilities	3,495	2,147
Total other long-term liabilities	3,495	2,147
STOCKHOLDERS' INVESTMENT	14,993	15,277
LIABILITIES AND STOCKHOLDERS' INVESTMENT	28,358	26,059
Guarantor Subsidiaries [Member]
CURRENT ASSETS
Cash and cash equivalents	487	441	405	417
Receivables, less allowances	4,383	4,338
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	689	674
Deferred income taxes	571	501
Total current assets	6,130	5,954
PROPERTY AND EQUIPMENT, AT COST	40,364	38,303
Less accumulated depreciation and amortization	20,685	19,899
Net property and equipment	19,679	18,404
INTERCOMPANY RECEIVABLE	686	2,366
GOODWILL	1,552	1,552
INVESTMENT IN SUBSIDIARIES	3,800	3,745
Other assets	898	747
ASSETS	32,745	32,768
CURRENT LIABILITIES
Current portion of long-term debt	7	1
Accrued salaries and employee benefits	1,208	1,042
Accounts payable	1,433	1,530
Accrued expenses	1,557	1,444
Total current liabilities	4,205	4,017
LONG-TERM DEBT, LESS CURRENT PORTION	248	249
OTHER LONG-TERM LIABILITIES
Deferred income taxes	4,206	4,059
Other liabilities	3,367	3,230
Total other long-term liabilities	7,573	7,289
STOCKHOLDERS' INVESTMENT	20,719	21,213
LIABILITIES AND STOCKHOLDERS' INVESTMENT	32,745	32,768
Non Guarantor Subsidiaries [Member]
CURRENT ASSETS
Cash and cash equivalents	971	861	717	636
Receivables, less allowances	1,385	1,151
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	123	60
Deferred income taxes	35	21
Total current assets	2,514	2,093
PROPERTY AND EQUIPMENT, AT COST	2,471	2,360
Less accumulated depreciation and amortization	1,281	1,220
Net property and equipment	1,190	1,140
INTERCOMPANY RECEIVABLE	1,563	1,320
GOODWILL	2,258	1,238
Other assets	477	250
ASSETS	8,002	6,041
CURRENT LIABILITIES
Current portion of long-term debt	12
Accrued salaries and employee benefits	194	180
Accounts payable	758	620
Accrued expenses	275	214
Total current liabilities	1,239	1,014
LONG-TERM DEBT, LESS CURRENT PORTION	23
OTHER LONG-TERM LIABILITIES
Deferred income taxes	225	93
Other liabilities	261	254
Total other long-term liabilities	486	347
STOCKHOLDERS' INVESTMENT	6,254	4,680
LIABILITIES AND STOCKHOLDERS' INVESTMENT	8,002	6,041
Consolidation Eliminations [Member]
CURRENT ASSETS
Cash and cash equivalents	(78)	(150)	$ (97)	$ (116)
Receivables, less allowances	(52)	(31)
Total current assets	(130)	(181)
INTERCOMPANY RECEIVABLE	(2,249)	(3,686)
INVESTMENT IN SUBSIDIARIES	(26,973)	(25,893)
Other assets	(2,684)	(2,038)
ASSETS	(32,036)	(31,798)
CURRENT LIABILITIES
Accounts payable	(130)	(181)
Total current liabilities	(130)	(181)
INTERCOMPANY PAYABLE	(2,249)	(3,686)
OTHER LONG-TERM LIABILITIES
Deferred income taxes	(2,684)	(2,038)
Total other long-term liabilities	(2,684)	(2,038)
STOCKHOLDERS' INVESTMENT	(26,973)	(25,893)
LIABILITIES AND STOCKHOLDERS' INVESTMENT	$ (32,036)	$ (31,798)