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BUSINESS SEGMENTS - Summary of Segment Revenue, Segment Expenses, and Segment Profits (Details) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2025 USD ($) segment | Sep. 30, 2024 USD ($)
Segment Reporting [Abstract] |  |  |  |
Number of reporting segments | segment |  |  | 4 |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | $ 6,489 | $ 6,255 | $ 19,496 | $ 18,671
Depreciation, amortization, and other | 50 | 45 | 154 | 137
General, administrative, and other | 234 | 276 | 724 | 785
OPERATING INCOME | 1,180 | 944 | 3,364 | 3,015
Total reportable segment revenue |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 5,743 | 5,603 | 17,379 | 16,752
Total reportable segment revenue | U.S. & Canada |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 4,741 | 4,596 | 14,401 | 13,851
Depreciation, amortization, and other | 23 | 21 | 77 | 63
General, administrative, and other | 40 | 59 | 120 | 145
Other segment items (primarily non-operating income and expenses) | (1) | (3) | (4) | (5)
OPERATING INCOME | 680 | 617 | 2,110 | 2,029
Total reportable segment revenue | EMEA |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 613 | 635 | 1,782 | 1,775
Depreciation, amortization, and other | 9 | 9 | 28 | 28
General, administrative, and other | 32 | 25 | 99 | 80
Other segment items (primarily non-operating income and expenses) | (2) | 0 | (2) | (3)
OPERATING INCOME | 158 | 152 | 389 | 386
Total reportable segment revenue | Greater China |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 142 | 142 | 430 | 430
Depreciation, amortization, and other | 5 | 2 | 10 | 7
General, administrative, and other | 15 | 14 | 46 | 41
Other segment items (primarily non-operating income and expenses) | (1) | 1 | (2) | 0
OPERATING INCOME | 44 | 46 | 142 | 144
Total reportable segment revenue | APEC |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 247 | 230 | 766 | 696
Depreciation, amortization, and other | 2 | 2 | 6 | 6
General, administrative, and other | 19 | 15 | 53 | 47
Other segment items (primarily non-operating income and expenses) | 0 | 0 | 1 | (2)
OPERATING INCOME | 63 | 66 | 219 | 200
Net fee revenues |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Gross fee revenues | 1,338 | 1,283 | 4,013 | 3,836
Contract investment amortization | (29) | (26) | (86) | (76)
Revenue | 1,309 | 1,257 | 3,927 | 3,760
Net fee revenues | Total reportable segment revenue | U.S. & Canada |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Gross fee revenues | 740 | 747 | 2,249 | 2,227
Contract investment amortization | (19) | (19) | (60) | (57)
Revenue | 721 | 728 | 2,189 | 2,170
Net fee revenues | Total reportable segment revenue | EMEA |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Gross fee revenues | 172 | 153 | 458 | 425
Contract investment amortization | (5) | (3) | (13) | (10)
Revenue | 167 | 150 | 445 | 415
Net fee revenues | Total reportable segment revenue | Greater China |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Gross fee revenues | 64 | 62 | 188 | 186
Contract investment amortization | (1) | 0 | (1) | 0
Revenue | 63 | 62 | 187 | 186
Net fee revenues | Total reportable segment revenue | APEC |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Gross fee revenues | 86 | 82 | 267 | 243
Contract investment amortization | (1) | (2) | (4) | (4)
Revenue | 85 | 80 | 263 | 239
Owned, leased, and other revenue |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 420 | 381 | 1,222 | 1,133
Operating costs | 326 | 300 | 950 | 882
Owned, leased, and other revenue | Total reportable segment revenue | U.S. & Canada |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 130 | 95 | 390 | 314
Operating costs | 96 | 82 | 292 | 240
Owned, leased, and other revenue | Total reportable segment revenue | EMEA |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 161 | 169 | 439 | 444
Operating costs | 132 | 132 | 373 | 369
Owned, leased, and other revenue | Total reportable segment revenue | Greater China |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 4 | 5 | 21 | 18
Operating costs | 5 | 3 | 13 | 10
Owned, leased, and other revenue | Total reportable segment revenue | APEC |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 32 | 30 | 110 | 98
Operating costs | 32 | 27 | 93 | 84
Cost reimbursement revenue |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 4,760 | 4,617 | 14,347 | 13,778
Operating costs | 4,739 | 4,681 | 14,335 | 13,827
Cost reimbursement revenue | Total reportable segment revenue | U.S. & Canada |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 3,890 | 3,773 | 11,822 | 11,367
Operating costs | 3,903 | 3,820 | 11,806 | 11,379
Cost reimbursement revenue | Total reportable segment revenue | EMEA |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 285 | 316 | 898 | 916
Operating costs | 284 | 317 | 895 | 915
Cost reimbursement revenue | Total reportable segment revenue | Greater China |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 75 | 75 | 222 | 226
Operating costs | 74 | 76 | 221 | 228
Cost reimbursement revenue | Total reportable segment revenue | APEC |  |  |  |
Segment Reporting, Revenue Reconciling Item [Line Items] |  |  |  |
Revenue | 130 | 120 | 393 | 359
Operating costs | $ 131 | $ 120 | $ 394 | $ 361

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