Condensed Consolidating Financial Statements (Details 3) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Nov. 30, 2016
Nov. 30, 2015
Nov. 30, 2016
Nov. 30, 2015
Condensed Financial Statements Captions [Line Items]        
REVENUES $ 14,931 $ 12,453 $ 29,594 $ 24,732
OPERATING EXPENSES:        
Salaries and employee benefits 5,353 4,570 10,664 9,095
Purchased transportation 3,431 2,538 6,671 4,882
Rentals and landing fees 802 682 1,592 1,377
Depreciation and amortization 740 653 1,479 1,301
Fuel 658 615 1,308 1,327
Maintenance and repairs 579 529 1,177 1,077
Other 2,201 1,729 4,272 3,392
OPERATING EXPENSES 13,764 11,316 27,163 22,451
OPERATING INCOME 1,167 1,137 2,431 2,281
OTHER INCOME (EXPENSE):        
Interest, net (119) (74) (232) (137)
Other, net 30 (8) 21 (5)
INCOME BEFORE INCOME TAXES 1,078 1,055 2,220 2,139
PROVISION FOR INCOME TAXES 378 364 805 756
NET INCOME 700 691 1,415 1,383
COMPREHENSIVE INCOME 451 640 1,159 1,170
Consolidation Eliminations Member        
Condensed Financial Statements Captions [Line Items]        
REVENUES (70) (74) (140) (177)
OPERATING EXPENSES:        
Purchased transportation (26) (25) (54) (80)
Rentals and landing fees (2) (2) (3) (3)
Other (42) (47) (83) (94)
OPERATING EXPENSES (70) (74) (140) (177)
OTHER INCOME (EXPENSE):        
Equity in earnings of subsidiaries (754) (764) (1,525) (1,517)
INCOME BEFORE INCOME TAXES (754) (764) (1,525) (1,517)
NET INCOME (754) (764) (1,525) (1,517)
COMPREHENSIVE INCOME (754) (764) (1,525) (1,517)
Parent Company Member        
OPERATING EXPENSES:        
Salaries and employee benefits 29 26 65 60
Rentals and landing fees 2 2 3 3
Depreciation and amortization   1   1
Intercompany charges, net (89) (112) (179) (181)
Other 58 83 111 117
OTHER INCOME (EXPENSE):        
Equity in earnings of subsidiaries 700 691 1,415 1,383
Interest, net (123) (81) (245) (156)
Intercompany charges, net 124 84 246 162
Other, net (1) (3) (1) (6)
INCOME BEFORE INCOME TAXES 700 691 1,415 1,383
NET INCOME 700 691 1,415 1,383
COMPREHENSIVE INCOME 682 672 1,378 1,346
Guarantor Subsidiaries Member        
Condensed Financial Statements Captions [Line Items]        
REVENUES 10,997 10,479 21,900 20,352
OPERATING EXPENSES:        
Salaries and employee benefits 4,161 3,926 8,267 7,739
Purchased transportation 2,074 1,941 3,991 3,375
Rentals and landing fees 625 596 1,245 1,183
Depreciation and amortization 634 601 1,245 1,184
Fuel 584 597 1,162 1,288
Maintenance and repairs 504 497 1,030 1,005
Intercompany charges, net 38 84 100 44
Other 1,429 1,293 2,802 2,557
OPERATING EXPENSES 10,049 9,535 19,842 18,375
OPERATING INCOME 948 944 2,058 1,977
OTHER INCOME (EXPENSE):        
Equity in earnings of subsidiaries 54 73 110 134
Interest, net 4 6 13 14
Intercompany charges, net (64) (83) (145) (159)
Other, net (5) (6) (10) (9)
INCOME BEFORE INCOME TAXES 937 934 2,026 1,957
PROVISION FOR INCOME TAXES 291 309 671 666
NET INCOME 646 625 1,355 1,291
COMPREHENSIVE INCOME 635 620 1,337 1,271
Non Guarantor Subsidiaries Member        
Condensed Financial Statements Captions [Line Items]        
REVENUES 4,004 2,048 7,834 4,557
OPERATING EXPENSES:        
Salaries and employee benefits 1,163 618 2,332 1,296
Purchased transportation 1,383 622 2,734 1,587
Rentals and landing fees 177 86 347 194
Depreciation and amortization 106 51 234 116
Fuel 74 18 146 39
Maintenance and repairs 75 32 147 72
Intercompany charges, net 51 28 79 137
Other 756 400 1,442 812
OPERATING EXPENSES 3,785 1,855 7,461 4,253
OPERATING INCOME 219 193 373 304
OTHER INCOME (EXPENSE):        
Interest, net   1   5
Intercompany charges, net (60) (1) (101) (3)
Other, net 36 1 32 10
INCOME BEFORE INCOME TAXES 195 194 304 316
PROVISION FOR INCOME TAXES 87 55 134 90
NET INCOME 108 139 170 226
COMPREHENSIVE INCOME $ (112) $ 112 $ (31) $ 70