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Guarantee of Registered Securities - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Cash flows from operating activities |  |  |
Cash provided by (used for) operating activities - continuing operations | $ 1,835 | $ 1,686 | $ 669
Cash provided by operating activities - discontinued operations | 0 | 0 | 65
Cash provided by (used for) operating activities | 1,835 | 1,686 | 734
Cash flows from investing activities |  |  |
Proceeds from investments | 61 | 71 | 68
Payments for investments | (113) | (80) | (64)
Net sales (purchases) of short-term investments — non-fiduciary | 35 | 348 | (232)
Acquisition of businesses, net of cash acquired | (39) | (58) | (1,029)
Sale of businesses, net of cash sold | 52 | (10) | 4,246
Capital expenditures | (225) | (240) | (183)
Cash provided by (used for) investing activities - continuing operations | (229) | 31 | 2,806
Cash used for investing activities - discontinued operations | 0 | 0 | (19)
Cash provided by (used for) investing activities | (229) | 31 | 2,787
Cash flows from financing activities |  |  |
Share repurchase | (1,960) | (1,470) | (2,399)
Advances from (to) affiliates | 0 | 0 | 0
Issuance of shares for employee benefit plans | (131) | (149) | (121)
Issuance of debt | 6,052 | 5,754 | 1,654
Repayment of debt | (4,941) | (5,417) | (1,999)
Cash dividends to shareholders | (410) | (382) | (364)
Noncontrolling interests and other financing activities | (103) | (35) | (36)
Cash provided by (used for) financing activities - continuing operations | (1,493) | (1,699) | (3,265)
Cash used for financing activities - discontinued operations | 0 | 0 | 0
Cash used for financing activities | (1,493) | (1,699) | (3,265)
Effect of exchange rates on cash and cash equivalents | 21 | (118) | 69
Net increase (decrease) in cash and cash equivalents | 134 | (100) | 325
Cash and cash equivalents at beginning of period | 656 | 756 | 431
Cash and cash equivalents at end of period | 790 | 656 | 756
Aon Ireland |  |  |
Cash flows from operating activities |  |  |
Cash provided by (used for) operating activities - continuing operations | 0 | 0 | 0
Cash provided by operating activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) operating activities | 0 | 0 | 0
Cash flows from investing activities |  |  |
Proceeds from investments | 0 | 0 | 0
Payments for investments | 0 | 0 | 0
Net sales (purchases) of short-term investments — non-fiduciary | 0 | 0 | 0
Acquisition of businesses, net of cash acquired | 0 | 0 | 0
Sale of businesses, net of cash sold | 0 | 0 | 0
Capital expenditures | 0 | 0 | 0
Cash provided by (used for) investing activities - continuing operations | 0 | 0 | 0
Cash used for investing activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) investing activities | 0 | 0 | 0
Cash flows from financing activities |  |  |
Share repurchase | 0 | 0 | 0
Advances from (to) affiliates | 0 | 0 | 0
Issuance of shares for employee benefit plans | 0 | 0 | 0
Issuance of debt | 0 | 0 | 0
Repayment of debt | 0 | 0 | 0
Cash dividends to shareholders | 0 | 0 | 0
Noncontrolling interests and other financing activities | 0 | 0 | 0
Cash provided by (used for) financing activities - continuing operations | 0 | 0 | 0
Cash used for financing activities - discontinued operations | 0 | 0 | 0
Cash used for financing activities | 0 | 0 | 0
Effect of exchange rates on cash and cash equivalents | 0 | 0 | 0
Net increase (decrease) in cash and cash equivalents | 0 | 0 | 0
Cash and cash equivalents at beginning of period | 0 | 0 | 0
Cash and cash equivalents at end of period | 0 | 0 | 0
Aon UK |  |  |
Cash flows from operating activities |  |  |
Cash provided by (used for) operating activities - continuing operations | 280 | 1,575 | 2,787
Cash provided by operating activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) operating activities | 280 | 1,575 | 2,787
Cash flows from investing activities |  |  |
Proceeds from investments | 0 | 0 | 224
Payments for investments | 0 | (13) | (261)
Net sales (purchases) of short-term investments — non-fiduciary | 0 | 0 | 0
Acquisition of businesses, net of cash acquired | 0 | 0 | 0
Sale of businesses, net of cash sold | 0 | 0 | 0
Capital expenditures | 0 | 0 | 0
Cash provided by (used for) investing activities - continuing operations | 0 | (13) | (37)
Cash used for investing activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) investing activities | 0 | (13) | (37)
Cash flows from financing activities |  |  |
Share repurchase | (1,960) | (1,470) | (2,399)
Advances from (to) affiliates | 2,350 | 156 | 426
Issuance of shares for employee benefit plans | (131) | (149) | (121)
Issuance of debt | 2,739 | 1,723 | 544
Repayment of debt | (2,868) | (1,441) | (835)
Cash dividends to shareholders | (410) | (382) | (364)
Noncontrolling interests and other financing activities | 0 | 0 | 0
Cash provided by (used for) financing activities - continuing operations | (280) | (1,563) | (2,749)
Cash used for financing activities - discontinued operations | 0 | 0 | 0
Cash used for financing activities | (280) | (1,563) | (2,749)
Effect of exchange rates on cash and cash equivalents | 0 | 0 | 0
Net increase (decrease) in cash and cash equivalents | 0 | (1) | 1
Cash and cash equivalents at beginning of period | 0 | 1 | 0
Cash and cash equivalents at end of period | 0 | 0 | 1
Aon Global Holdings Limited |  |  |
Cash flows from operating activities |  |  |
Cash provided by (used for) operating activities - continuing operations | 1,346 | 2,676 | 618
Cash provided by operating activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) operating activities | 1,346 | 2,676 | 618
Cash flows from investing activities |  |  |
Proceeds from investments | 123 | 0 | 1,683
Payments for investments | 0 | 0 | (1,141)
Net sales (purchases) of short-term investments — non-fiduciary | 0 | 0 | 0
Acquisition of businesses, net of cash acquired | 0 | 0 | 0
Sale of businesses, net of cash sold | 0 | 0 | 0
Capital expenditures | 0 | 0 | 0
Cash provided by (used for) investing activities - continuing operations | 123 | 0 | 542
Cash used for investing activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) investing activities | 123 | 0 | 542
Cash flows from financing activities |  |  |
Share repurchase | (1,469) | 0 | 0
Advances from (to) affiliates | 0 | (2,676) | (1,160)
Issuance of shares for employee benefit plans | 0 | 0 | 0
Issuance of debt | 0 | 0 | 0
Repayment of debt | 0 | 0 | 0
Cash dividends to shareholders | 0 | 0 | 0
Noncontrolling interests and other financing activities | 0 | 0 | 0
Cash provided by (used for) financing activities - continuing operations | (1,469) | (2,676) | (1,160)
Cash used for financing activities - discontinued operations | 0 | 0 | 0
Cash used for financing activities | (1,469) | (2,676) | (1,160)
Effect of exchange rates on cash and cash equivalents | 0 | 0 | 0
Net increase (decrease) in cash and cash equivalents | 0 | 0 | 0
Cash and cash equivalents at beginning of period | 0 | 0 | 0
Cash and cash equivalents at end of period | 0 | 0 | 0
Aon Corporation |  |  |
Cash flows from operating activities |  |  |
Cash provided by (used for) operating activities - continuing operations | (140) | 3 | 503
Cash provided by operating activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) operating activities | (140) | 3 | 503
Cash flows from investing activities |  |  |
Proceeds from investments | 29 | 24 | 587
Payments for investments | (62) | (47) | (29)
Net sales (purchases) of short-term investments — non-fiduciary | 28 | 299 | (215)
Acquisition of businesses, net of cash acquired | 0 | 0 | 0
Sale of businesses, net of cash sold | 0 | 0 | 0
Capital expenditures | 0 | 0 | 0
Cash provided by (used for) investing activities - continuing operations | (5) | 276 | 343
Cash used for investing activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) investing activities | (5) | 276 | 343
Cash flows from financing activities |  |  |
Share repurchase | 0 | 0 | 0
Advances from (to) affiliates | 314 | (2,291) | 95
Issuance of shares for employee benefit plans | 0 | 0 | 0
Issuance of debt | 3,313 | 4,028 | 1,100
Repayment of debt | (2,073) | (3,678) | (1,150)
Cash dividends to shareholders | 0 | 0 | 0
Noncontrolling interests and other financing activities | 0 | 0 | 0
Cash provided by (used for) financing activities - continuing operations | 1,554 | (1,941) | 45
Cash used for financing activities - discontinued operations | 0 | 0 | 0
Cash used for financing activities | 1,554 | (1,941) | 45
Effect of exchange rates on cash and cash equivalents | 0 | 0 | 0
Net increase (decrease) in cash and cash equivalents | 1,409 | (1,662) | 891
Cash and cash equivalents at beginning of period | 862 | 2,524 | 1,633
Cash and cash equivalents at end of period | 2,271 | 862 | 2,524
Other Non-Guarantor Subsidiaries |  |  |
Cash flows from operating activities |  |  |
Cash provided by (used for) operating activities - continuing operations | 2,254 | 2,700 | 1,804
Cash provided by operating activities - discontinued operations | 0 | 0 | 65
Cash provided by (used for) operating activities | 2,254 | 2,700 | 1,869
Cash flows from investing activities |  |  |
Proceeds from investments | 189 | 955 | 582
Payments for investments | (124) | (33) | (2,259)
Net sales (purchases) of short-term investments — non-fiduciary | 7 | 49 | (17)
Acquisition of businesses, net of cash acquired | (39) | (58) | (1,029)
Sale of businesses, net of cash sold | 52 | (10) | 4,246
Capital expenditures | (225) | (240) | (183)
Cash provided by (used for) investing activities - continuing operations | (140) | 663 | 1,340
Cash used for investing activities - discontinued operations | 0 | 0 | (19)
Cash provided by (used for) investing activities | (140) | 663 | 1,321
Cash flows from financing activities |  |  |
Share repurchase | 0 | 0 | 0
Advances from (to) affiliates | (2,886) | (2,523) | (1,966)
Issuance of shares for employee benefit plans | 0 | 0 | 0
Issuance of debt | 0 | 3 | 10
Repayment of debt | 0 | (298) | (14)
Cash dividends to shareholders | 0 | 0 | 0
Noncontrolling interests and other financing activities | (103) | (35) | (36)
Cash provided by (used for) financing activities - continuing operations | (2,989) | (2,853) | (2,006)
Cash used for financing activities - discontinued operations | 0 | 0 | 0
Cash used for financing activities | (2,989) | (2,853) | (2,006)
Effect of exchange rates on cash and cash equivalents | 21 | (118) | 69
Net increase (decrease) in cash and cash equivalents | (854) | 392 | 1,253
Cash and cash equivalents at beginning of period | 3,473 | 3,081 | 1,828
Cash and cash equivalents at end of period | 2,619 | 3,473 | 3,081
Consolidating Adjustments |  |  |
Cash flows from operating activities |  |  |
Cash provided by (used for) operating activities - continuing operations | (1,905) | (5,268) | (5,043)
Cash provided by operating activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) operating activities | (1,905) | (5,268) | (5,043)
Cash flows from investing activities |  |  |
Proceeds from investments | (280) | (908) | (3,008)
Payments for investments | 73 | 13 | 3,626
Net sales (purchases) of short-term investments — non-fiduciary | 0 | 0 | 0
Acquisition of businesses, net of cash acquired | 0 | 0 | 0
Sale of businesses, net of cash sold | 0 | 0 | 0
Capital expenditures | 0 | 0 | 0
Cash provided by (used for) investing activities - continuing operations | (207) | (895) | 618
Cash used for investing activities - discontinued operations | 0 | 0 | 0
Cash provided by (used for) investing activities | (207) | (895) | 618
Cash flows from financing activities |  |  |
Share repurchase | 0 | 0 | 0
Advances from (to) affiliates | 1,691 | 7,334 | 2,605
Issuance of shares for employee benefit plans | 0 | 0 | 0
Issuance of debt | 0 | 0 | 0
Repayment of debt | 0 | 0 | 0
Cash dividends to shareholders | 0 | 0 | 0
Noncontrolling interests and other financing activities | 0 | 0 | 0
Cash provided by (used for) financing activities - continuing operations | 1,691 | 7,334 | 2,605
Cash used for financing activities - discontinued operations | 0 | 0 | 0
Cash used for financing activities | 1,691 | 7,334 | 2,605
Effect of exchange rates on cash and cash equivalents | 0 | 0 | 0
Net increase (decrease) in cash and cash equivalents | (421) | 1,171 | (1,820)
Cash and cash equivalents at beginning of period | (3,679) | (4,850) | (3,030)
Cash and cash equivalents at end of period | $ (4,100) | $ (3,679) | $ (4,850)

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- Definition Amount of cash outflow in the form of ordinary dividends to common shareholders of the parent entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
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- Definition Amount of cash outflow to satisfy grantee's tax withholding obligation for award under share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
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- Definition The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585 + Details
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- Definition Amount of cash outflow to acquire investments classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585 + Details
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- Definition The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585 + Details
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- Definition Amount of cash outflow to a noncontrolling interest. Includes, but not limited to, reduction of noncontrolling interest ownership. Excludes dividends paid to the noncontrolling interest. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
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- Definition This element represents the cash inflow during the period from the sale of a component of the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585 + Details
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- Definition The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585 + Details
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- Definition The cash inflow associated with the sale and maturity (principal being due) of other investments, prepayment and call (request of early payment) of other investments not otherwise defined in the taxonomy. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585 + Details
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- Definition The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585 + Details
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