Condensed Consolidating Financial Statements (Details 2) (USD $) In Millions, unless otherwise specified	Feb. 28, 2015	May 31, 2014	Feb. 28, 2014	May 31, 2013
CURRENT ASSETS
Cash and cash equivalents	$ 3,478	$ 2,908	$ 3,042	$ 4,917
Receivables, less allowances	5,584	5,460
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	910	793
Deferred income taxes	510	522
Total current assets	10,482	9,683
PROPERTY AND EQUIPMENT, AT COST	42,652	40,691
Less accumulated depreciation and amortization	22,227	21,141
Net property and equipment	20,425	19,550
GOODWILL	3,805	2,790
Other assets	1,396	1,047
ASSETS	36,108	33,070
CURRENT LIABILITIES
Current portion of long-term debt		1
Accrued salaries and employee benefits	1,231	1,277
Accounts payable	2,050	1,971
Accrued expenses	1,962	2,063
Total current liabilities	5,243	5,312
LONG-TERM DEBT, LESS CURRENT PORTION	7,228	4,736
OTHER LONG-TERM LIABILITIES
Deferred income taxes	2,497	2,114
Other liabilities	5,128	5,631
Total other long-term liabilities	7,625	7,745
STOCKHOLDERS' INVESTMENT	16,012	15,277
LIABILITIES AND STOCKHOLDERS' INVESTMENT	36,108	33,070
Parent Company Member
CURRENT ASSETS
Cash and cash equivalents	2,142	1,756	2,016	3,892
Receivables, less allowances	1	2
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	100	59
Total current assets	2,243	1,817
PROPERTY AND EQUIPMENT, AT COST	28	28
Less accumulated depreciation and amortization	23	22
Net property and equipment	5	6
INVESTMENT IN SUBSIDIARIES	23,736	20,785
Other assets	2,055	2,088
ASSETS	28,039	24,696
CURRENT LIABILITIES
Accrued salaries and employee benefits	35	55
Accounts payable	60	2
Accrued expenses	314	405
Total current liabilities	409	462
LONG-TERM DEBT, LESS CURRENT PORTION	6,978	4,487
INTERCOMPANY PAYABLE	2,984	2,323
OTHER LONG-TERM LIABILITIES
Other liabilities	1,656	2,147
Total other long-term liabilities	1,656	2,147
STOCKHOLDERS' INVESTMENT	16,012	15,277
LIABILITIES AND STOCKHOLDERS' INVESTMENT	28,039	24,696
Guarantor Subsidiaries Member
CURRENT ASSETS
Cash and cash equivalents	492	441	380	405
Receivables, less allowances	4,355	4,338
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	687	674
Deferred income taxes	476	501
Total current assets	6,010	5,954
PROPERTY AND EQUIPMENT, AT COST	40,264	38,303
Less accumulated depreciation and amortization	20,973	19,899
Net property and equipment	19,291	18,404
INTERCOMPANY RECEIVABLE	1,406	1,058
GOODWILL	1,552	1,552
INVESTMENT IN SUBSIDIARIES	3,753	3,754
Other assets	856	747
ASSETS	32,868	31,469
CURRENT LIABILITIES
Current portion of long-term debt		1
Accrued salaries and employee benefits	1,053	1,042
Accounts payable	1,327	1,530
Accrued expenses	1,410	1,444
Total current liabilities	3,790	4,017
LONG-TERM DEBT, LESS CURRENT PORTION	248	249
OTHER LONG-TERM LIABILITIES
Deferred income taxes	4,294	4,059
Other liabilities	3,229	3,230
Total other long-term liabilities	7,523	7,289
STOCKHOLDERS' INVESTMENT	21,307	19,914
LIABILITIES AND STOCKHOLDERS' INVESTMENT	32,868	31,469
Non Guarantor Subsidiaries Member
CURRENT ASSETS
Cash and cash equivalents	886	861	788	717
Receivables, less allowances	1,277	1,151
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	123	60
Deferred income taxes	34	21
Total current assets	2,320	2,093
PROPERTY AND EQUIPMENT, AT COST	2,360	2,360
Less accumulated depreciation and amortization	1,231	1,220
Net property and equipment	1,129	1,140
INTERCOMPANY RECEIVABLE	1,578	1,265
GOODWILL	2,253	1,238
Other assets	473	250
ASSETS	7,753	5,986
CURRENT LIABILITIES
Accrued salaries and employee benefits	143	180
Accounts payable	754	620
Accrued expenses	238	214
Total current liabilities	1,135	1,014
LONG-TERM DEBT, LESS CURRENT PORTION	2
OTHER LONG-TERM LIABILITIES
Deferred income taxes	191	93
Other liabilities	243	254
Total other long-term liabilities	434	347
STOCKHOLDERS' INVESTMENT	6,182	4,625
LIABILITIES AND STOCKHOLDERS' INVESTMENT	7,753	5,986
Consolidation Eliminations Member
CURRENT ASSETS
Cash and cash equivalents	(42)	(150)	(142)	(97)
Receivables, less allowances	(49)	(31)
Total current assets	(91)	(181)
INTERCOMPANY RECEIVABLE	(2,984)	(2,323)
INVESTMENT IN SUBSIDIARIES	(27,489)	(24,539)
Other assets	(1,988)	(2,038)
ASSETS	(32,552)	(29,081)
CURRENT LIABILITIES
Accounts payable	(91)	(181)
Total current liabilities	(91)	(181)
INTERCOMPANY PAYABLE	(2,984)	(2,323)
OTHER LONG-TERM LIABILITIES
Deferred income taxes	(1,988)	(2,038)
Total other long-term liabilities	(1,988)	(2,038)
STOCKHOLDERS' INVESTMENT	(27,489)	(24,539)
LIABILITIES AND STOCKHOLDERS' INVESTMENT	$ (32,552)	$ (29,081)