Condensed Consolidating Financial Statements (Details 2) - USD ($) $ in Millions	Aug. 31, 2015	May. 31, 2015	Aug. 31, 2014	May. 31, 2014
CURRENT ASSETS
Cash and cash equivalents	$ 3,543	$ 3,763	$ 2,416	$ 2,908
Receivables, less allowances	5,617	5,719
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	937	853
Deferred income taxes	606	606
Total current assets	10,703	10,941
PROPERTY AND EQUIPMENT, AT COST	43,989	42,864
Less accumulated depreciation and amortization	22,506	21,989
Net property and equipment	21,483	20,875
Goodwill	3,792	3,810
Other assets	1,267	1,443
ASSETS	37,245	37,069
CURRENT LIABILITIES
Current portion of long-term debt	14	19
Accrued salaries and employee benefits	1,355	1,436
Accounts payable	2,049	2,066
Accrued expenses	2,426	2,436
Total current liabilities	5,844	5,957
LONG-TERM DEBT, LESS CURRENT PORTION	7,244	7,249
OTHER LONG-TERM LIABILITIES
Deferred income taxes	1,781	1,747
Other liabilities	7,093	7,123
Total other long-term liabilities	8,874	8,870
STOCKHOLDERS' INVESTMENT	15,283	14,993
LIABILITIES AND STOCKHOLDERS' INVESTMENT	37,245	37,069
Parent Company Member
CURRENT ASSETS
Cash and cash equivalents	2,224	2,383	1,305	1,756
Receivables, less allowances	1	3
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	25	41
Total current assets	2,250	2,427
PROPERTY AND EQUIPMENT, AT COST	29	29
Less accumulated depreciation and amortization	23	23
Net property and equipment	6	6
INVESTMENT IN SUBSIDIARIES	23,722	23,173
Other assets	2,761	2,752
ASSETS	28,739	28,358
CURRENT LIABILITIES
Accrued salaries and employee benefits	44	34
Accounts payable	77	5
Accrued expenses	745	604
Total current liabilities	866	643
LONG-TERM DEBT, LESS CURRENT PORTION	6,978	6,978
INTERCOMPANY PAYABLE	2,228	2,249
OTHER LONG-TERM LIABILITIES
Other liabilities	3,384	3,495
Total other long-term liabilities	3,384	3,495
STOCKHOLDERS' INVESTMENT	15,283	14,993
LIABILITIES AND STOCKHOLDERS' INVESTMENT	28,739	28,358
Guarantor Subsidiaries Member
CURRENT ASSETS
Cash and cash equivalents	446	487	430	441
Receivables, less allowances	4,308	4,383
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	777	689
Deferred income taxes	572	571
Total current assets	6,103	6,130
PROPERTY AND EQUIPMENT, AT COST	41,521	40,364
Less accumulated depreciation and amortization	21,202	20,685
Net property and equipment	20,319	19,679
INTERCOMPANY RECEIVABLE	533	686
Goodwill	1,551	1,552
INVESTMENT IN SUBSIDIARIES	3,817	3,800
Other assets	781	898
ASSETS	33,104	32,745
CURRENT LIABILITIES
Current portion of long-term debt	6	7
Accrued salaries and employee benefits	1,133	1,208
Accounts payable	1,326	1,433
Accrued expenses	1,400	1,557
Total current liabilities	3,865	4,205
LONG-TERM DEBT, LESS CURRENT PORTION	248	248
OTHER LONG-TERM LIABILITIES
Deferred income taxes	4,237	4,206
Other liabilities	3,446	3,367
Total other long-term liabilities	7,683	7,573
STOCKHOLDERS' INVESTMENT	21,308	20,719
LIABILITIES AND STOCKHOLDERS' INVESTMENT	33,104	32,745
Non Guarantor Subsidiaries Member
CURRENT ASSETS
Cash and cash equivalents	917	971	821	861
Receivables, less allowances	1,349	1,385
Spare parts, supplies, fuel, prepaid expenses and other, less allowances	135	123
Deferred income taxes	34	35
Total current assets	2,435	2,514
PROPERTY AND EQUIPMENT, AT COST	2,439	2,471
Less accumulated depreciation and amortization	1,281	1,281
Net property and equipment	1,158	1,190
INTERCOMPANY RECEIVABLE	1,695	1,563
Goodwill	2,241	2,258
Other assets	416	477
ASSETS	7,945	8,002
CURRENT LIABILITIES
Current portion of long-term debt	8	12
Accrued salaries and employee benefits	178	194
Accounts payable	731	758
Accrued expenses	281	275
Total current liabilities	1,198	1,239
LONG-TERM DEBT, LESS CURRENT PORTION	18	23
OTHER LONG-TERM LIABILITIES
Deferred income taxes	235	225
Other liabilities	263	261
Total other long-term liabilities	498	486
STOCKHOLDERS' INVESTMENT	6,231	6,254
LIABILITIES AND STOCKHOLDERS' INVESTMENT	7,945	8,002
Consolidation Eliminations Member
CURRENT ASSETS
Cash and cash equivalents	(44)	(78)	$ (140)	$ (150)
Receivables, less allowances	(41)	(52)
Total current assets	(85)	(130)
INTERCOMPANY RECEIVABLE	(2,228)	(2,249)
INVESTMENT IN SUBSIDIARIES	(27,539)	(26,973)
Other assets	(2,691)	(2,684)
ASSETS	(32,543)	(32,036)
CURRENT LIABILITIES
Accounts payable	(85)	(130)
Total current liabilities	(85)	(130)
INTERCOMPANY PAYABLE	(2,228)	(2,249)
OTHER LONG-TERM LIABILITIES
Deferred income taxes	(2,691)	(2,684)
Total other long-term liabilities	(2,691)	(2,684)
STOCKHOLDERS' INVESTMENT	(27,539)	(26,973)
LIABILITIES AND STOCKHOLDERS' INVESTMENT	$ (32,543)	$ (32,036)