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Guarantee of Registered Securities - Condensed Consolidating Statement of Income (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Revenue |  |  |  |  |  |  |  |  |  |  |
Total revenue | $ 2,885 | $ 2,379 | $ 2,606 | $ 3,143 | $ 2,770 | $ 2,349 | $ 2,561 | $ 3,090 | $ 11,013 | $ 10,770 | $ 9,998
Expenses |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits |  |  |  |  |  |  |  |  | 6,054 | 6,103 | 6,003
Information technology |  |  |  |  |  |  |  |  | 494 | 484 | 419
Premises |  |  |  |  |  |  |  |  | 339 | 370 | 348
Depreciation of fixed assets |  |  |  |  |  |  |  |  | 172 | 176 | 187
Amortization and impairment of intangible assets |  |  |  |  |  |  |  |  | 392 | 593 | 704
Other general expense |  |  |  |  |  |  |  |  | 1,393 | 1,500 | 1,272
Total operating expenses |  |  |  |  |  |  |  |  | 8,844 | 9,226 | 8,933
Operating income | 524 | 360 | 413 | 872 | 499 | 262 | (16) | 799 | 2,169 | 1,544 | 1,065
Interest income |  |  |  |  |  |  |  |  | 8 | 5 | 27
Interest expense |  |  |  |  |  |  |  |  | (307) | (278) | (282)
Intercompany interest income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Intercompany other income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other income (expense) |  |  |  |  |  |  |  |  | 1 | (25) | (125)
Income from continuing operations before income taxes |  |  |  |  |  |  |  |  | 1,871 | 1,246 | 685
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 297 | 146 | 250
Net income from continuing operations | 382 | 229 | 287 | 676 | 284 | 155 | 57 | 604 | 1,574 | 1,100 | 435
Net income (loss) from discontinued operations | 0 | (1) | 0 | 0 | 69 | (2) | 1 | 6 | (1) | 74 | 828
Net income (loss) before equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 1,573 | 1,174 | 1,263
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income | 382 | 228 | 287 | 676 | 353 | 153 | 58 | 610 | 1,573 | 1,174 | 1,263
Less: Net income attributable to noncontrolling interests | 8 | 6 | 10 | 17 | 8 | 6 | 10 | 16 | 41 | 40 | 37
Net income attributable to Aon shareholders | $ 374 | $ 222 | $ 277 | $ 659 | $ 345 | $ 147 | $ 48 | $ 594 | 1,532 | 1,134 | 1,226
Aon Ireland |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 0 | 0 | 0
Expenses |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits |  |  |  |  |  |  |  |  | 0 | 0 | 0
Information technology |  |  |  |  |  |  |  |  | 0 | 0 | 0
Premises |  |  |  |  |  |  |  |  | 0 | 0 | 0
Depreciation of fixed assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization and impairment of intangible assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other general expense |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total operating expenses |  |  |  |  |  |  |  |  | 0 | 0 | 0
Operating income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Interest income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Interest expense |  |  |  |  |  |  |  |  | 0 | 0 | 0
Intercompany interest income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Intercompany other income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Income from continuing operations before income taxes |  |  |  |  |  |  |  |  | 0 | 0 | 0
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income from continuing operations |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income (loss) before equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 0 | 0 | 0
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 1,540 | 0 | 0
Net income |  |  |  |  |  |  |  |  | 1,540 | 0 | 0
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 1,540 | 0 | 0
Aon UK |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 0 | 0 | 0
Expenses |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits |  |  |  |  |  |  |  |  | 49 | 74 | 150
Information technology |  |  |  |  |  |  |  |  | 0 | 0 | 0
Premises |  |  |  |  |  |  |  |  | 0 | 0 | 0
Depreciation of fixed assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization and impairment of intangible assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other general expense |  |  |  |  |  |  |  |  | 10 | 4 | 12
Total operating expenses |  |  |  |  |  |  |  |  | 59 | 78 | 162
Operating income |  |  |  |  |  |  |  |  | (59) | (78) | (162)
Interest income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Interest expense |  |  |  |  |  |  |  |  | (188) | (203) | (202)
Intercompany interest income (expense) |  |  |  |  |  |  |  |  | 21 | 15 | 14
Intercompany other income (expense) |  |  |  |  |  |  |  |  | 185 | 65 | 282
Other income (expense) |  |  |  |  |  |  |  |  | 18 | 41 | (62)
Income from continuing operations before income taxes |  |  |  |  |  |  |  |  | (23) | (160) | (130)
Income tax expense (benefit) |  |  |  |  |  |  |  |  | (12) | (60) | (43)
Net income from continuing operations |  |  |  |  |  |  |  |  | (11) | (100) | (87)
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income (loss) before equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | (11) | (100) | (87)
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 1,551 | 1,255 | 1,295
Net income |  |  |  |  |  |  |  |  | 1,540 | 1,155 | 1,208
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 1,540 | 1,155 | 1,208
Aon Global Holdings Limited |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 0 | 0 | 0
Expenses |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits |  |  |  |  |  |  |  |  | 0 | 0 | 0
Information technology |  |  |  |  |  |  |  |  | 0 | 0 | 0
Premises |  |  |  |  |  |  |  |  | 0 | 0 | 0
Depreciation of fixed assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization and impairment of intangible assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other general expense |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total operating expenses |  |  |  |  |  |  |  |  | 0 | 0 | 0
Operating income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Interest income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Interest expense |  |  |  |  |  |  |  |  | (81) | (83) | (33)
Intercompany interest income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 1
Intercompany other income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Income from continuing operations before income taxes |  |  |  |  |  |  |  |  | (81) | (83) | (32)
Income tax expense (benefit) |  |  |  |  |  |  |  |  | (8) | (15) | (6)
Net income from continuing operations |  |  |  |  |  |  |  |  | (73) | (68) | (26)
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income (loss) before equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | (73) | (68) | (26)
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 283 | 100 | 457
Net income |  |  |  |  |  |  |  |  | 210 | 32 | 431
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 210 | 32 | 431
Aon Corporation |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 0 | 0 | 0
Expenses |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits |  |  |  |  |  |  |  |  | 20 | (1) | 26
Information technology |  |  |  |  |  |  |  |  | 0 | 0 | 0
Premises |  |  |  |  |  |  |  |  | 23 | 0 | 0
Depreciation of fixed assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization and impairment of intangible assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other general expense |  |  |  |  |  |  |  |  | 4 | 90 | 20
Total operating expenses |  |  |  |  |  |  |  |  | 47 | 89 | 46
Operating income |  |  |  |  |  |  |  |  | (47) | (89) | (46)
Interest income |  |  |  |  |  |  |  |  | 39 | 58 | 52
Interest expense |  |  |  |  |  |  |  |  | (133) | (101) | (94)
Intercompany interest income (expense) |  |  |  |  |  |  |  |  | (464) | (514) | (543)
Intercompany other income (expense) |  |  |  |  |  |  |  |  | (362) | (373) | (385)
Other income (expense) |  |  |  |  |  |  |  |  | (60) | (48) | 12
Income from continuing operations before income taxes |  |  |  |  |  |  |  |  | (1,027) | (1,067) | (1,004)
Income tax expense (benefit) |  |  |  |  |  |  |  |  | (182) | (192) | (110)
Net income from continuing operations |  |  |  |  |  |  |  |  | (845) | (875) | (894)
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income (loss) before equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | (845) | (875) | (894)
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 1,254 | 1,009 | 1,141
Net income |  |  |  |  |  |  |  |  | 409 | 134 | 247
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 409 | 134 | 247
Other Non-Guarantor Subsidiaries |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 11,013 | 10,770 | 9,998
Expenses |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits |  |  |  |  |  |  |  |  | 5,985 | 6,030 | 5,827
Information technology |  |  |  |  |  |  |  |  | 494 | 484 | 419
Premises |  |  |  |  |  |  |  |  | 316 | 370 | 348
Depreciation of fixed assets |  |  |  |  |  |  |  |  | 172 | 176 | 187
Amortization and impairment of intangible assets |  |  |  |  |  |  |  |  | 392 | 593 | 704
Other general expense |  |  |  |  |  |  |  |  | 1,379 | 1,406 | 1,240
Total operating expenses |  |  |  |  |  |  |  |  | 8,738 | 9,059 | 8,725
Operating income |  |  |  |  |  |  |  |  | 2,275 | 1,711 | 1,273
Interest income |  |  |  |  |  |  |  |  | 67 | 60 | 37
Interest expense |  |  |  |  |  |  |  |  | (3) | (4) | (15)
Intercompany interest income (expense) |  |  |  |  |  |  |  |  | 443 | 499 | 528
Intercompany other income (expense) |  |  |  |  |  |  |  |  | 177 | 308 | 103
Other income (expense) |  |  |  |  |  |  |  |  | 51 | 3 | (93)
Income from continuing operations before income taxes |  |  |  |  |  |  |  |  | 3,010 | 2,577 | 1,833
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 499 | 413 | 409
Net income from continuing operations |  |  |  |  |  |  |  |  | 2,511 | 2,164 | 1,424
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  | (1) | 74 | 828
Net income (loss) before equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 2,510 | 2,238 | 2,252
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income |  |  |  |  |  |  |  |  | 2,510 | 2,238 | 2,252
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 41 | 40 | 37
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 2,469 | 2,198 | 2,215
Consolidating Adjustments |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 0 | 0 | 0
Expenses |  |  |  |  |  |  |  |  |  |  |
Compensation and benefits |  |  |  |  |  |  |  |  | 0 | 0 | 0
Information technology |  |  |  |  |  |  |  |  | 0 | 0 | 0
Premises |  |  |  |  |  |  |  |  | 0 | 0 | 0
Depreciation of fixed assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization and impairment of intangible assets |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other general expense |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total operating expenses |  |  |  |  |  |  |  |  | 0 | 0 | 0
Operating income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Interest income |  |  |  |  |  |  |  |  | (98) | (113) | (62)
Interest expense |  |  |  |  |  |  |  |  | 98 | 113 | 62
Intercompany interest income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Intercompany other income (expense) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other income (expense) |  |  |  |  |  |  |  |  | (8) | (21) | 18
Income from continuing operations before income taxes |  |  |  |  |  |  |  |  | (8) | (21) | 18
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income from continuing operations |  |  |  |  |  |  |  |  | (8) | (21) | 18
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income (loss) before equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | (8) | (21) | 18
Equity in earnings of subsidiaries |  |  |  |  |  |  |  |  | (4,628) | (2,364) | (2,893)
Net income |  |  |  |  |  |  |  |  | (4,636) | (2,385) | (2,875)
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | $ (4,636) | $ (2,385) | $ (2,875)

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