Aon (AON) 10-K 14 Feb 20 2019 FY Annual report Financial data

Cover

Cover - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 13, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	1-7933
Entity Registrant Name	Aon plc
Entity Incorporation, State or Country Code	X0
Entity Tax Identification Number	98-1030901
Entity Address, Address Line One	122 LEADENHALL STREET
Entity Address, City or Town	LONDON
Entity Address, Country	GB
Entity Address, Postal Zip Code	EC3V 4AN
City Area Code	20
Local Phone Number	7623 5500
Title of 12(b) Security	Class A Ordinary Shares, $0.01 nominal value
Trading Symbol	AON
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 45,489,377,212
Entity Common Stock, Shares Outstanding		231,582,094
Documents Incorporated by Reference	Aon plc has announced a transaction that, if completed, would result in an Irish public limited company becoming a successor issuer to Aon plc for purposes of Rule 12g-3 under the Securities Exchange Act of 1934, as amended. The proxy statement for the 2020 annual general meeting of shareholders of Aon plc, or if the transaction is completed, of such successor issuer, which meetings are in either case scheduled to be held on June 19, 2020 , are incorporated by reference in this report in response to Part III, Items 10, 11, 12, 13 and 14.
Entity Central Index Key	0000315293
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ 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)
Other income (expense)									1	(25)	(125)
Income from continuing operations before income taxes									1,871	1,246	685
Income tax expense									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	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
Basic net income per share attributable to Aon shareholders
Basic net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 1.59	$ 0.94	$ 1.15	$ 2.72	$ 1.14	$ 0.61	$ 0.19	$ 2.37	$ 6.42	$ 4.32	$ 1.54
Basic net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	0	0	0	0	0.28	(0.01)	0.01	0.02	0	0.30	3.20
Basic net income per share attributable to Aon shareholders (in dollars per share)	1.59	0.94	1.15	2.72	1.42	0.60	0.20	2.39	6.42	4.62	4.74
Diluted net income per share attributable to Aon shareholders
Diluted net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	1.58	0.93	1.14	2.70	1.13	0.61	0.19	2.35	6.37	4.29	1.53
Diluted net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	0	0	0	0	0.28	(0.01)	0	0.02	0	0.30	3.17
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ 1.58	$ 0.93	$ 1.14	$ 2.70	$ 1.41	$ 0.60	$ 0.19	$ 2.37	$ 6.37	$ 4.59	$ 4.70
Weighted average ordinary shares outstanding - basic (in shares)									238.6	245.2	258.5
Weighted average ordinary shares outstanding - diluted (in shares)									240.6	247	260.7

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - 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
Statement of Comprehensive Income [Abstract]
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
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									3	11	12
Foreign currency translation adjustments									14	(444)	390
Postretirement benefit obligation									(141)	17	19
Total other comprehensive income (loss)									(124)	(416)	421
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	(4)	5
Total other comprehensive income (loss) attributable to Aon shareholders									(124)	(412)	416
Comprehensive income attributable to Aon shareholders									$ 1,408	$ 722	$ 1,642

Consolidated Statements of Fina

Consolidated Statements of Financial Position - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 790	$ 656
Short-term investments	138	172
Receivables, net	3,112	2,760
Fiduciary assets	11,834	10,166
Other current assets	602	618
Total current assets	16,476	14,372
Goodwill	8,165	8,171
Intangible assets, net	783	1,149
Fixed assets, net	621	588
Operating lease right-of-use assets	929
Deferred tax assets	645	561
Prepaid pension	1,216	1,133
Other non-current assets	570	448
Total assets	29,405	26,422
Current liabilities
Accounts payable and accrued liabilities	1,939	1,943
Short-term debt and current portion of long-term debt	712	251
Fiduciary liabilities	11,834	10,166
Other current liabilities	1,086	936
Total current liabilities	15,571	13,296
Long-term debt	6,627	5,993
Non-current operating lease liabilities	944
Deferred tax liabilities	199	181
Pension, other postretirement, and postemployment liabilities	1,738	1,636
Other non-current liabilities	877	1,097
Total liabilities	25,956	22,203
Equity
Ordinary shares - $0.01 nominal value Authorized: 750 shares (issued: 2019 - 232.1; 2018 - 240.1)	2	2
Additional paid-in capital	6,152	5,965
Retained earnings	1,254	2,093
Accumulated other comprehensive loss	(4,033)	(3,909)
Total Aon shareholders' equity	3,375	4,151
Noncontrolling interests	74	68
Total equity	3,449	4,219
Total liabilities and equity	$ 29,405	$ 26,422

Consolidated Statements of Fi_2

Consolidated Statements of Financial Position (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Ordinary shares, nominal value (in dollars per share)	$ 0.01	$ 0.01
Ordinary shares, authorized shares (in Shares)	750,000,000	750,000,000
Ordinary shares, issued shares (in Shares)	232,100,000	240,100,000

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Ordinary Shares and Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss, Net of Tax	Non-controlling Interests
Beginning Balance (in shares) at Dec. 31, 2016		262,000,000
Beginning Balance at Dec. 31, 2016	$ 5,581	$ 5,580	$ 3,856	$ (3,912)	$ 57
Increase (Decrease) in Shareholders' Equity
Net income	1,263		1,226		37
Shares issued - employee stock compensation plans (in shares)		3,600,000
Shares issued - employee stock compensation plans	(121)	$ (120)	(1)
Shares purchased (in shares)		(18,000,000)
Shares purchased	(2,415)		(2,415)
Share-based compensation expense	321	$ 321
Dividends to shareholders	(364)		(364)
Net change in fair value of financial instruments	12			12
Net foreign currency translation adjustments	390			385	5
Net postretirement benefit obligation	19			19
Purchases of shares from noncontrolling interests	(11)	$ (4)			(7)
Dividends paid to noncontrolling interests on subsidiary common stock	(27)				(27)
Ending Balance (in shares) at Dec. 31, 2017		247,600,000
Ending Balance at Dec. 31, 2017	4,648	$ 5,777	2,302	(3,496)	65
Increase (Decrease) in Shareholders' Equity
Net income	1,174		1,134		40
Shares issued - employee stock compensation plans (in shares)		2,500,000
Shares issued - employee stock compensation plans	(148)	$ (148)
Shares purchased (in shares)		(10,000,000)
Shares purchased	(1,454)		(1,454)
Share-based compensation expense	338	$ 338
Dividends to shareholders	(382)		(382)
Net change in fair value of financial instruments	11			11
Net foreign currency translation adjustments	(444)			(440)	(4)
Net postretirement benefit obligation	17			17
Purchases of shares from noncontrolling interests	(1)				(1)
Dividends paid to noncontrolling interests on subsidiary common stock	$ (32)				(32)
Ending Balance (in shares) at Dec. 31, 2018	240,100,000	240,100,000
Ending Balance at Dec. 31, 2018	$ 4,219	$ 5,967	2,093	(3,909)	68
Increase (Decrease) in Shareholders' Equity
Net income	1,573		1,532		41
Shares issued - employee stock compensation plans (in shares)		2,500,000
Shares issued - employee stock compensation plans	(131)	$ (130)	(1)
Shares purchased (in shares)		(10,500,000)
Shares purchased	(1,960)		(1,960)
Share-based compensation expense	317	$ 317
Dividends to shareholders	(410)		(410)
Net change in fair value of financial instruments	3			3
Net foreign currency translation adjustments	14			14
Net postretirement benefit obligation	(141)			(141)
Dividends paid to noncontrolling interests on subsidiary common stock	$ (35)				(35)
Ending Balance (in shares) at Dec. 31, 2019	232,100,000	232,100,000
Ending Balance at Dec. 31, 2019	$ 3,449	$ 6,154	$ 1,254	$ (4,033)	$ 74

Consolidated Statements of Sh_2

Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Stockholders' Equity [Abstract]
Dividends (in dollars per share)	$ 1.72	$ 1.56	$ 1.41

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 1,573	$ 1,174	$ 1,263
Net income (loss) from discontinued operations	(1)	74	828
Adjustments to reconcile net income to cash provided by operating activities:
Loss (gain) from sales of businesses and investments, net	(13)	6	16
Depreciation of fixed assets	172	176	187
Amortization and impairment of intangible assets	392	593	704
Share-based compensation expense	317	338	319
Deferred income taxes	(36)	(225)	(18)
Change in assets and liabilities:
Fiduciary receivables	(409)	(679)	171
Short-term investments — funds held on behalf of clients	(1,246)	(320)	(135)
Fiduciary liabilities	1,655	999	(36)
Receivables, net	(371)	(127)	(254)
Accounts payable and accrued liabilities	(28)	25	96
Restructuring reserves	3	23	172
Current income taxes	(20)	34	(914)
Pension, other postretirement and postemployment liabilities	(156)	(259)	(66)
Other assets and liabilities	1	2	(8)
Cash provided by 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)
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
Supplemental disclosures:
Interest paid	289	266	272
Income taxes paid, net of refunds	$ 353	$ 337	$ 1,182

Basis of Presentation

Basis of Presentation	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying Consolidated Financial Statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”). The Consolidated Financial Statements include the accounts of Aon plc and all of its controlled subsidiaries (“Aon” or the “Company”). Intercompany accounts and transactions have been eliminated. The Consolidated Financial Statements include, in the opinion of management, all adjustments (consisting of normal recurring adjustments and reclassifications) necessary to present fairly the Company’s consolidated financial position, results of operations and cash flows for all periods presented. Use of Estimates The preparation of the accompanying Consolidated Financial Statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosures of contingent assets and liabilities at the date of the Consolidated Financial Statements, and the reported amounts of reserves and expenses. These estimates and assumptions are based on management’s best estimates and judgments. Management evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors, including the current economic environment. Management believes its estimates to be reasonable given the current facts available. Aon adjusts such estimates and assumptions when facts and circumstances dictate. Illiquid credit markets, volatile equity markets, and foreign currency exchange rate movements increase the uncertainty inherent in such estimates and assumptions. As future events and their effects cannot be determined with precision, actual results could differ significantly from these estimates. Changes in estimates resulting from continuing changes in the economic environment would, if applicable, be reflected in the Consolidated Financial Statements in future periods.

Summary of Significant Accounti

Summary of Significant Accounting Principles and Practices	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Principles and Practices	Summary of Significant Accounting Principles and Practices Revenue Recognition The Company generates revenues primarily through commissions, compensation from insurance and reinsurance companies for services provided to them, and fees from customers. Commissions and fees for brokerage services vary depending upon several factors, which may include the amount of premium, the type of insurance or reinsurance coverage provided, the particular services provided to a client, insurer, or reinsurer, and the capacity in which the Company acts. Compensation from insurance and reinsurance companies includes: (1) fees for consulting and analytics services and (2) fees and commissions for administrative and other services provided to or on behalf of insurers. In Aon’s capacity as an insurance and reinsurance broker, the service promised to the customer is placement of an effective insurance or reinsurance policy, respectively. At the completion of the insurance or reinsurance policy placement process once coverage is effective, the customer has obtained control over the services promised by the Company. Judgment is not typically required when assessing whether the coverage is effective. Fees from clients for advice and consulting services are dependent on the extent and value of the services provided. Payment terms for the Company’s principal service lines are discussed below; the Company believes these terms are consistent with current industry practices. Significant financing components are typically not present in Aon’s arrangements. The Company recognizes revenue when control of the promised services is transferred to the customer in the amount that best reflects the consideration to which the Company expects to be entitled in exchange for those services. For arrangements where control is transferred over time, an input or output method is applied that represents a faithful depiction of the progress towards completion of the performance obligation. For arrangements that include variable consideration, the Company assesses whether any amounts should be constrained. For arrangements that include multiple performance obligations, the Company allocates consideration based on their relative fair values. Costs incurred by the Company in obtaining a contract are capitalized and amortized on a systematic basis that is consistent with the transfer of control of the services to which the asset relates, considering anticipated renewals when applicable. Certain contract related costs, including pre-placement brokerage costs, are capitalized as a cost to fulfill and are amortized on a systematic basis consistent with the transfer of control of the services to which the asset relates, which is generally less than one year. The Company has elected to apply practical expedients to not disclose the revenue related to unsatisfied performance obligations if (1) the contract has an original duration of 1 year or less, (2) the Company has recognized revenue for the amount in which it has the right to bill, and (3) the variable consideration is allocated entirely to an unsatisfied performance obligation which is recognized as a series of distinct goods or services that form a single performance obligation. Disaggregation of Revenue The following is a description of principal service lines from which the Company generates its revenue: Commercial Risk Solutions includes retail brokerage, cyber solutions, global risk consulting, and captives. Revenue primarily includes insurance commissions and fees for services rendered. Revenue is predominantly recognized at a point in time upon the effective date of the underlying policy (or policies), or for a limited number of arrangements, over the term of the arrangement using output measures to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services. For arrangements recognized over time, various output measures, including units transferred and time elapsed, are utilized to provide a faithful depiction of the progress towards completion of the performance obligation. Revenue is recorded net of allowances for estimated policy cancellations, which are determined based on an evaluation of historical and current cancellation data. Commissions and fees for brokerage services may be invoiced near the effective date of the underlying policy or over the term of the arrangement in installments during the policy period. Reinsurance Solutions includes treaty and facultative reinsurance brokerage and capital markets. Revenue primarily includes reinsurance commissions and fees for services rendered. Revenue is predominantly recognized at a point in time upon the effective date of the underlying policy (or policies), or for a limited number of arrangements, over the term of the arrangement using output measures to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services. For arrangements recognized over time, various output measures, including units delivered and time elapsed, are utilized to provide a faithful depiction of the progress towards completion of the performance obligation. Commissions and fees for brokerage services may be invoiced at the inception of the reinsurance period for certain reinsurance brokerage, or more commonly, over the term of the arrangement in installments based on deposit or minimum premiums for most treaty reinsurance arrangements. Retirement Solutions includes core retirement, investment consulting, and human capital. Revenue consists primarily of fees paid by customers for consulting services, such as risk management strategies, health and benefits, and human capital consulting services. Revenue recognized for these arrangements is predominantly recognized over the term of the arrangement using input or output measures to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services, or for certain arrangements, at a point in time upon completion of the services. For consulting arrangements recognized over time, revenue will be recognized based on a measure of progress that depicts the transfer of control of the services to the customer, utilizing an appropriate input or output measure to provide a reasonable assessment of the progress towards completion of the performance obligation including units delivered or time elapsed. Fees paid by customers for consulting services are typically charged on an hourly, project or fixed-fee basis, and revenue for these arrangements is typically recognized based on time incurred, days elapsed, or reports delivered. Revenue from time-and-materials or cost-plus arrangements are recognized as services are performed using input or output measures to provide a reasonable assessment of the progress towards completion of the performance obligation including hours worked, and revenue for these arrangements is typically recognized based on time and materials incurred. Reimbursements received for out-of-pocket expenses are recorded as a component of revenue. Payment terms vary but are typically over the contract term in installments. Health Solutions includes health and benefits brokerage and health care exchanges. Revenue primarily includes insurance commissions and fees for services rendered. For brokerage commissions, revenue is predominantly recognized at the effective date of the underlying policy (or policies), or for a limited number of arrangements, over the term of the arrangement to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services using input or output measures, including units delivered or time elapsed, to provide a faithful depiction of the progress towards completion of the performance obligation. Revenue from health care exchange arrangements are typically recognized upon successful enrollment of participants, net of a reserve for estimated cancellations. Commissions and fees for brokerage services may be invoiced at the effective date of the underlying policy or over the term of the arrangement in installments during the policy period. Payment terms for other services vary but are typically over the contract term in installments. Data & Analytic Services includes Affinity, Aon InPoint, and ReView. Revenue consists primarily of fees for services rendered and is generally recognized over the term of the arrangement to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services. Payment terms vary but are typically over the contract term in installments. For arrangements recognized over time, revenue will be recognized based on a measure of progress that depicts the transfer of control of the services to the customer, utilizing an appropriate input or output measure to provide a faithful depiction of the progress towards completion of the performance obligation, including units delivered or time elapsed. Input and output measures utilized vary based on the arrangement but typically include reports provided or days elapsed. Share-based Compensation Expense Share-based payments to employees, including grants of restricted share units and performance share awards, are measured based on grant date fair value. The Company recognizes compensation expense over the requisite service period for awards expected to ultimately vest. Forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates. Pension and Other Postretirement Benefits The Company records net periodic cost relating to its pension and other postretirement benefit plans based on calculations that include various actuarial assumptions, including discount rates, assumed rates of return on plan assets, inflation rates, mortality rates, compensation increases, and turnover rates. The Company reviews its actuarial assumptions on an annual basis and modifies these assumptions based on current rates and trends. The effects of gains, losses, and prior service costs and credits are amortized over future service periods or future estimated lives if the plans are frozen as reflected in Other income (expense) within the Consolidated Statements of Income. The funded status of each plan, calculated as the fair value of plan assets less the benefit obligation, is reflected in the Company’s Consolidated Statements of Financial Position using a December 31 measurement date. Net Income per Share Basic net income per share is computed by dividing net income available to ordinary shareholders by the weighted-average number of ordinary shares outstanding, including participating securities, which consist of unvested share awards with non-forfeitable rights to dividends. Diluted net income per share is computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding, which have been adjusted for the dilutive effect of potentially issuable ordinary shares, including certain contingently issuable shares. The diluted earnings per share calculation reflects the more dilutive effect of either (1) the two-class method that assumes that the participating securities have not been exercised, or (2) the treasury stock method. Potentially issuable shares are not included in the computation of diluted income per share if their inclusion would be antidilutive. Cash and Cash Equivalents and Short-term Investments Cash and cash equivalents include cash balances and all highly liquid investments with initial maturities of three months or less. Short-term investments consist of money market funds. The estimated fair value of Cash and cash equivalents and Short-term investments approximates their carrying values. At December 31, 2019 , Cash and cash equivalents and Short-term investments totaled $928 million compared to $828 million at December 31, 2018 . Of the total balance, $110 million and $91 million was restricted as to its use at December 31, 2019 and 2018 , respectively. Included within the December 31, 2019 and 2018 balances, respectively, were £42.7 million ( $55.5 million at December 31, 2019 exchanges rates) and £42.7 million ( $53.9 million at December 31, 2018 exchange rates) of operating funds required to be held by the Company in the U.K. by the FCA, which were included in Short-term investments. Fiduciary Assets and Liabilities In its capacity as an insurance agent and broker, Aon collects premiums from insureds and, after deducting its commission, remits the premiums to the respective insurers. Aon also collects claims or refunds from insurers on behalf of insureds. Uncollected premiums from insureds and uncollected claims or refunds from insurers are recorded as Fiduciary assets in the Company’s Consolidated Statements of Financial Position. Unremitted insurance premiums and claims are held in a fiduciary capacity and the obligation to remit these funds is recorded as Fiduciary liabilities in the Consolidated Statements of Financial Position. Aon held fiduciary assets for premiums collected from insureds but not yet remitted to insurance companies and claims collected from insurance companies but not yet remitted to insureds of $5.2 billion and $3.9 billion at December 31, 2019 and 2018 , respectively. These funds and a corresponding liability are included in Fiduciary assets and Fiduciary liabilities, respectively, in the accompanying Consolidated Statements of Financial Position. Allowance for Doubtful Accounts The Company’s allowance for doubtful accounts with respect to receivables is based on a combination of factors, including evaluation of historical write-offs, aging of balances, and other qualitative and quantitative analyses. Receivables, net included an allowance for doubtful accounts of $70 million and $62 million at December 31, 2019 and 2018 , respectively. Fixed Assets Fixed assets are stated at cost, less accumulated depreciation. Included in this category are certain capitalized costs incurred during the application development stage related to directly obtaining, developing, or enhancing internal use software. Depreciation and amortization are computed using the straight-line method over the estimated useful lives of the assets, which are generally as follows: Asset Description Estimated Useful Life Software Lesser of the life of an associated license, or 4 to 7 years Leasehold improvements Lesser of estimated useful life or lease term, not to exceed 10 years Furniture, fixtures and equipment 4 to 10 years Computer equipment 4 to 6 years Buildings 35 years Automobiles 6 years Goodwill and Intangible Assets Goodwill represents the excess of acquisition cost over the fair value of the net assets acquired in the acquisition of a business. Goodwill is allocated to applicable reporting units. Upon disposition of a business entity, goodwill is allocated to the disposed entity based on the fair value of that entity compared to the fair value of the reporting unit in which it was included. Goodwill is not amortized, but instead is tested for impairment at least annually. The goodwill impairment test is performed at the reporting unit level. The Company may initially perform a qualitative analysis to determine if it is more likely than not that the goodwill balance is impaired. If a qualitative assessment is not performed or if a determination is made that it is not more likely than not that their value of the reporting unit exceeds its carrying amount, then the Company will perform a two-step quantitative analysis. First, the fair value of each reporting unit is compared to its carrying value. If the fair value of the reporting unit is less than its carrying value, the Company performs a hypothetical purchase price allocation based on the reporting unit’s fair value to determine the fair value of the reporting unit’s goodwill. Any resulting difference will be a charge to Amortization and impairment of intangible assets in the Consolidated Statements of Income in the period in which the determination is made. Fair value is determined using a combination of present value techniques and market prices of comparable businesses. We classify our intangible assets acquired as either tradenames, customer-related and contract-based, or technology and other. Amortization basis and estimated useful lives by intangible asset type are generally as follows: Intangible Asset Description Amortization Basis Estimated Useful Life Tradenames Straight-line 1 to 3 years Customer-related and contract-based In line with underlying cash flows 7 to 20 years Technology and other Straight-line 5 to 7 years Derivatives Derivative instruments are recognized in the Consolidated Statements of Financial Position at fair value. Where the Company has entered into master netting agreements with counterparties, the derivative positions are netted by counterparties and are reported accordingly in other assets or other liabilities. Changes in the fair value of derivative instruments are recognized in earnings each period, unless the derivative is designated and qualifies as a cash flow or net investment hedge. The Company has historically designated the following hedging relationships for certain transactions: (1) a hedge of the change in fair value of a recognized asset or liability or firm commitment (“fair value hedge”), (2) a hedge of the variability in cash flows from a recognized variable-rate asset or liability or forecasted transaction (“cash flow hedge”), and (3) a hedge of the net investment in a foreign operation (“net investment hedge”). In order for a derivative to qualify for hedge accounting, the derivative must be formally designated as a fair value, cash flow, or a net investment hedge by documenting the relationship between the derivative and the hedged item. The documentation must include a description of the hedging instrument, the hedged item, the risk being hedged, Aon’s risk management objective and strategy for undertaking the hedge, and the method for assessing the effectiveness of the hedge. Additionally, the hedge relationship must be expected to be highly effective at offsetting changes in either the fair value or cash flows of the hedged item at both the inception of the hedge and on an ongoing basis. Aon assesses the ongoing effectiveness of its hedges quarterly or more frequently if facts and circumstances require. For a derivative designated as a fair value hedging instrument, the gain or loss is recognized in earnings in the period of change together with the offsetting loss or gain on the hedged item attributable to the risk being hedged. The effect is to reflect in earnings the extent to which the hedge is not effective in achieving offsetting changes in fair value. For a cash flow hedge that qualifies for hedge accounting, the change in fair value of a hedging instrument is recognized in Accumulated other comprehensive income (“AOCI”) and subsequently reclassified to earnings in the same period the hedged item impacts earnings. For a net investment hedge, the change in fair value of the hedging instrument is recognized in AOCI as part of the cumulative translation adjustment. Changes in the fair value of a derivative that is not designated as part of a hedging relationship (commonly referred to as an “economic hedge”) are recorded in Other income (expense) in the Consolidated Statements of Income in the period of change. The Company discontinues hedge accounting prospectively when (1) the derivative expires or is sold, terminated, or exercised, (2) the qualifying criteria are no longer met, or (3) management removes the designation of the hedging relationship. Foreign Currency Certain of the Company’s non-U.S. operations use their respective local currency as their functional currency. These operations that do not have the U.S. dollar as their functional currency translate their financial statements at the current rates of exchange in effect at the balance sheet date and revenues and expenses using rates that approximate those in effect during the period. The resulting translation adjustments are included in Net foreign currency translation adjustments within the Consolidated Statements of Shareholders’ Equity. Further, gains and losses from the remeasurement of monetary assets and liabilities that are denominated in a non-functional currency of that entity are included in Other income (expense) within the Consolidated Statements of Income. Income Taxes Deferred income taxes are recognized for the effect of temporary differences between financial reporting and tax basis of assets and liabilities and are measured using the enacted marginal tax rates and laws that are currently in effect. The effect on deferred tax assets and liabilities from a change in tax rates is recognized in the period when the rate change is enacted. Deferred tax assets are reduced by valuation allowances if, based on the consideration of all available evidence, it is more likely than not that some portion of the deferred tax asset will not be realized. Deferred tax assets are realized by having sufficient future taxable income to allow the related tax benefits to reduce taxes otherwise payable. The sources of taxable income that may be available to realize the benefit of deferred tax assets are future reversals of existing taxable temporary differences, future taxable income exclusive of reversing temporary differences and carry-forwards, taxable income in carry-back years, and tax planning strategies that are both prudent and feasible. The Company recognizes the effect of income tax positions only if sustaining those positions is more likely than not. Tax positions that meet the more likely than not recognition threshold but are not highly certain are initially and subsequently measured based on the largest amount of benefit that is greater than 50% likely of being realized upon settlement with the taxing authority. Only information that is available at the reporting date is considered in the Company’s recognition and measurement analysis, and events or changes in facts and circumstances are accounted for in the period in which the event or change in circumstance occurs. The Company records penalties and interest related to unrecognized tax benefits in Income taxes in the Company’s Consolidated Statements of Income. Leases The Company leases office facilities, equipment, and automobiles under non-cancelable operating and finance leases. The Company’s lease obligations are primarily for the use of office facilities. The Company evaluates if a leasing arrangement exists upon inception of a contract. A contract contains a lease if the contract conveys the right to control the use of identified tangible assets for a period of time in exchange for consideration. Identified property, plant, or equipment may include a physically distinct portion of a larger asset, or a portion of an asset that represents substantially all of the capacity of the asset but is not physically distinct. The Company assesses whether a contract implicitly contains the right to control the use of a tangible asset that is not already owned. In addition, the Company subleases certain real estate properties to third parties, which are classified as operating leases. The Company’s leases expire at various dates and may contain renewal and expansion options. The exercise of lease renewal and expansion options are typically at the Company’s sole discretion and are only included in the determination of the lease term if the Company is reasonably certain to exercise the option. The Company’s leases do not typically contain termination options. In addition, the Company’s lease agreements typically do not contain any material residual value guarantees or restrictive covenants. ROU assets and lease liabilities are based on the present value of the minimum lease payments over the lease term. The Company has elected the practical expedient related to lease and non-lease components, as an accounting policy election for all asset classes, which allows a lessee to not separate non-lease from lease components and instead account for consideration received in a contract as a single lease component. The Company made a policy election to not recognize ROU assets and lease liabilities that arise from leases with an initial term of twelve months or less on the Consolidated Statements of Financial Position. However, the Company recognized these lease payments in the Consolidated Statements of Income on a straight-line basis over the lease term and variable lease payments in the period in which the expense was incurred. The Company chose to apply this accounting policy across all classes of underlying assets. A portion of the Company’s lease agreements include variable lease payments that are not recorded in the initial measurement of the lease liability and ROU asset balances. For real estate arrangements, base rental payments may be escalated according to annual changes in the Consumer Price Index (“CPI”) or other indices. The escalated rental payments based on the estimated CPI at the lease commencement date are included within minimum rental payments; however, changes in CPI are considered variable in nature and are recognized as variable lease costs in the period in which the obligation is incurred. Additionally, real estate lease agreements may include other variable payments related to operating expenses charged by the landlord based on actual expenditures. Information technology equipment agreements may include variable payments based on usage of the equipment. These expenses are also recognized as variable lease costs in the period in which the expense is incurred. The Company utilizes discount rates to determine the present value of the lease payments based on information available at the commencement date of the lease. As the rate implicit in each lease is not typically readily available, the Company uses an incremental borrowing rate based on factors such as the lease term and the economic environment where the lease exists to determine the appropriate present value of future lease payments. When determining the incremental borrowing rate, the Company considers the rate of interest it would pay on a secured borrowing in an amount equal to the lease payments for the underlying asset under similar terms. Operating leases are included in Operating lease ROU assets, Other current liabilities, and Non-current operating lease liabilities on the Consolidated Statements of Financial Position. Finance leases are included in Other non-current assets, Other current liabilities, and Other non-current liabilities on the Consolidated Statements of Financial Position. Principles of Consolidation The accompanying Consolidated Financial Statements include the accounts of Aon plc and those entities in which the Company has a controlling financial interest. To determine if Aon holds a controlling financial interest in an entity, the Company first evaluates if it is required to apply the variable interest entity (“VIE”) model to the entity, otherwise, the entity is evaluated under the voting interest model. Where Aon holds rights that give it the power to direct the activities of a VIE that most significantly impact the VIE's economic performance, combined with a variable interest that gives the right to receive potentially significant benefits or the obligation to absorb potentially significant losses, the Company has a controlling financial interest in that VIE. Aon holds a controlling financial interest in entities that are not VIEs where it, directly or indirectly, holds more than 50% of the voting rights or where it exercises control through substantive participating rights or as a general partner. New Accounting Pronouncements Adoption of New Accounting Standards Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the Financial Accounting Standards Board (“FASB”) issued new accounting guidance related to reclassification of certain tax effects from accumulated other comprehensive income. The guidance allowed a reclassification from accumulated comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act of 2017. The guidance was effective for the Company in the first quarter of 2019. Aon did not elect to reclassify stranded tax effects on the Consolidated Statement of Financial Position; therefore, for the three and twelve months ended December 31, 2019 , there was no impact on the net income of the Company. It is the Company’s policy to release income tax effects from accumulated other comprehensive loss using the portfolio approach. Targeted Improvements to Accounting for Hedging Activities In August 2017, the FASB issued new accounting guidance related to targeted improvements to accounting for hedging activities. The new guidance amended its hedge accounting model to enable entities to better portray their risk management activities in the financial statements. The guidance eliminated the requirement to separately measure and report hedge ineffectiveness and required the effect of a hedging instrument to be presented in the same income statement line as the hedged item. The new guidance was effective for Aon in the first quarter of 2019 and the Company adopted it on a modified retrospective basis with no cumulative effect adjustment to Accumulated other comprehensive income (loss) or corresponding adjustment to Retained earnings. Changes to the Consolidated Statement of Income and financial statement disclosures were applied prospectively. Under the new guidance, gains or losses on certain derivative hedging instruments are recognized in Revenue, as opposed to Other income (expense) under the previous guidance. For the three and twelve months ended December 31, 2019 , the adoption of this guidance had no impact on the net income and an insignificant impact on the operating income of the Company. Leases In February 2016, the FASB issued a new accounting standard related to leases, which required lessees to recognize assets and liabilities for most leases. Under the new standard, a lessee is required to recognize in the Consolidated Statements of Financial Position, liabilities to make future lease payments and ROU assets representing its right to use the underlying assets for the lease term. The recognition, measurement, timing, and presentation of expenses and cash flows arising from a lease by a lessee did not significantly changed from previous U.S. GAAP. Under previous U.S. GAAP, leases classified as operating were not required to be recognized as assets and liabilities on the Consolidated Statements of Financial Position, and expense for minimum lease payments would be recognized on a straight-line basis over the term of the lease in the Consolidated Statements of Income. Leases classified as capital leases under previous U.S. GAAP were initially recorded using the present value of the minimum lease payments, with an associated capital lease liability, classified as short-term or long-term, as appropriate in the Consolidated Statements of Financial Position. The Company adopted the new standard as of January 1, 2019 using the modified retrospective approach for all leases existing at, or entered into after, the period of adoption. Under this approach, prior periods were not restated. Rather, lease balances and other disclosures for prior periods were provided in the Notes to Consolidate Financial Statements as previously reported, and the cumulative effect of initially applying the guidance was recognized in the Consolidated Statement of Financial Position as of December 31, 2019 . The modified retrospective approach included several optional practical expedients available that entities could elect to apply upon transition. These practical expedients related to the identification and classification of leases that commenced before the effective date, initial direct costs for leases that commenced before the effective date, and the ability to use hindsight in evaluating lessee options to extend or terminate a lease or to purchase the underlying asset. The Company elected the package of practical expedients permitted under the transition guidance within the n

Revenue from Contracts with Cus

Revenue from Contracts with Customers	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	Revenue from Contracts with Customers Disaggregation of Revenue The following table summarizes revenue from contracts with customers by principal service line (in millions): Years ended December 31 2019 2018 2017 Commercial Risk Solutions $ 4,673 $ 4,652 $ 4,169 Reinsurance Solutions 1,686 1,563 1,429 Retirement Solutions 1,817 1,865 1,755 Health Solutions 1,667 1,596 1,515 Data & Analytic Services 1,184 1,105 1,140 Elimination (14 ) (11 ) (10 ) Total revenue $ 11,013 $ 10,770 $ 9,998 Consolidated revenue from contracts with customers by geographic area, which is attributed on the basis of where the services are performed, is as follows (in millions): Years ended December 31 2019 2018 2017 United States $ 5,016 $ 4,677 $ 4,425 Americas other than United States 919 940 976 United Kingdom 1,502 1,555 1,436 Europe, Middle East, & Africa other than United Kingdom 2,338 2,413 2,025 Asia Pacific 1,238 1,185 1,136 Total revenue $ 11,013 $ 10,770 $ 9,998 Contract Costs Changes in the net carrying amount of costs to fulfill contracts with customers are as follows (in millions): 2019 2018 Balance at beginning of period $ 329 $ 298 Additions 1,453 1,504 Amortization (1,447 ) (1,465 ) Impairment — — Foreign currency translation and other — (8 ) Balance at end of period $ 335 $ 329 Changes in the net carrying amount of costs to obtain contracts with customers are as follows (in millions): 2019 2018 Balance at beginning of period $ 156 $ 145 Additions 58 53 Amortization (44 ) (41 ) Impairment — — Foreign currency translation and other 1 (1 ) Balance at end of period $ 171 $ 156

Other Financial Data

Other Financial Data	12 Months Ended
	Dec. 31, 2019
Other Financial Data [Abstract]
Other Financial Data	Other Financial Data Consolidated Statements of Income Information Other Income (Expense) The components of Other income (expense) are as follows (in millions): Years ended December 31 2019 2018 2017 Foreign currency remeasurement $ 9 $ 25 $ (37 ) Disposal of businesses 13 (6 ) (16 ) Pension and other postretirement 9 1 (86 ) Equity earnings 4 4 12 Financial instruments (34 ) (49 ) 2 Total $ 1 $ (25 ) $ (125 ) Consolidated Statements of Financial Position Information Allowance for Doubtful Accounts Changes in the net carrying amount of allowance for doubtful accounts are as follows (in millions): 2019 2018 2017 Balance at beginning of period $ 62 $ 59 $ 56 Provision 27 24 18 Accounts written off, net of recoveries (19 ) (25 ) (18 ) Foreign currency translation and other — 4 3 Balance at end of period $ 70 $ 62 $ 59 Other Current Assets The components of Other current assets are as follows (in millions): As of December 31 2019 2018 Costs to fulfill contracts with customers $ 335 $ 329 Taxes receivable 88 113 Prepaid expenses 97 97 Receivables from the Divested Business (1) 4 12 Other 78 67 Total $ 602 $ 618 (1) Refer to Note 5 “Discontinued Operations” for further information. Fixed Assets, net The components of Fixed assets, net are as follows (in millions): As of December 31 2019 2018 Software $ 727 $ 693 Leasehold improvements 358 334 Computer equipment 250 279 Furniture, fixtures and equipment 225 228 Construction in progress 183 154 Other 37 45 Fixed assets, gross 1,780 1,733 Less: Accumulated depreciation 1,159 1,145 Fixed assets, net $ 621 $ 588 Depreciation expense, which includes software amortization, was $172 million , $176 million , and $187 million for the years ended December 31, 2019 , 2018 , and 2017 , respectively. Other Non-current Assets The components of Other non-current assets are as follows (in millions): As of December 31 2019 2018 Costs to obtain contracts with customers $ 171 $ 156 Investments 53 54 Leases (1) 100 — Taxes receivable 102 100 Other 144 138 Total $ 570 $ 448 (1) In the first quarter of 2019, Aon adopted new accounting guidance related to the treatment of leases which was applied under the modified retrospective approach. The Other non-current asset balances related to leasing under the legacy accounting standard are reflected in Other as reported in December 31, 2018. Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for further information. Other Current Liabilities The components of Other current liabilities are as follows (in millions): As of December 31 2019 2018 Deferred revenue (1) $ 270 $ 251 Taxes payable 93 83 Leases (2) 210 — Other 513 602 Total $ 1,086 $ 936 (1) $532 million and $487 million was recognized in the Consolidated Statements of Income during the twelve months ended December 31, 2019 and December 31, 2018 , respectively. (2) In the first quarter of 2019, Aon adopted new accounting guidance related to the treatment of leases which was applied under the modified retrospective approach. The Other current liability balances related to leasing under the legacy accounting standard are reflected in Other as reported in December 31, 2018. Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for further information. Other Non-current Liabilities The components of Other non-current liabilities are as follows (in millions): As of December 31 2019 2018 Taxes payable (1) $ 525 $ 585 Leases (2) 76 169 Compensation and benefits 49 56 Deferred revenue 62 65 Other 165 222 Total $ 877 $ 1,097 (1) Includes $145 million and $240 million for the non-current portion of the Transition Tax, as of December 31, 2019 and December 31, 2018 , respectively. Refer to Note 11 “Income Taxes” for further information on the Transition Tax. (2) In the first quarter of 2019, Aon adopted new accounting guidance related to the treatment of leases which was applied under the modified retrospective approach. The comparable period presented reflects balances under the legacy accounting standard. Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for further information.

Discontinued Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2019
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations On February 9, 2017, the Company entered into a Purchase Agreement with Tempo Acquisition, LLC (the “Purchase Agreement”) to sell the Divested Business to an entity formed and controlled by affiliates of the Buyer and certain designated purchasers that are direct or indirect subsidiaries of the Buyer. On May 1, 2017, the Buyer purchased all of the outstanding equity interests of the Divested Business, plus certain related assets and liabilities, for a purchase price of $4.3 billion in cash paid at closing, subject to customary adjustments set forth in the Purchase Agreement, and deferred consideration of up to $500 million . Cash proceeds after customary adjustments and before taxes due were $4.2 billion . Aon and the Buyer entered into certain transaction-related agreements at the closing, including two commercial agreements, a transition services agreement, certain intellectual property license agreements, subleases, and other customary agreements. Aon expects to continue to be a significant client of the Divested Business and the Divested Business has agreed to use Aon for its broking and other services for a specified period of time. The financial results of the Divested Business for the years ended December 31, 2019 , 2018 , and 2017 are presented as Net income (loss) from discontinued operations on the Company’s Consolidated Statements of Income. The following table presents the financial results of the Divested Business (in millions): Years ended December 31 2019 2018 2017 Revenue Total revenue $ — $ — $ 698 Expenses Total operating expenses 2 12 656 Operating income from discontinued operations (2 ) (12 ) 42 Other income — — 10 Income (loss) from discontinued operations before income taxes (2 ) (12 ) 52 Income tax expense (benefit) (1 ) (4 ) 3 Net income (loss) from discontinued operations, excluding gain (1 ) (8 ) 49 Gain on sale of discontinued operations, net of tax — 82 779 Net income (loss) from discontinued operations $ (1 ) $ 74 $ 828 Upon triggering held for sale criteria in February 2017, Aon ceased depreciating and amortizing all long-lived assets included in discontinued operations. Total operating expenses for the year ended December 31, 2017, include $8 million of depreciation of fixed assets and $11 million of intangible asset amortization for the time prior to the Company triggering held for sale criteria. The Company’s Consolidated Statements of Cash Flows present the operating, investing, and financing cash flows of the Divested Business as discontinued operations. Aon uses a centralized approach to cash management and financing of its operations. Prior to the closing of the transaction, portions of the Divested Business’s cash were transferred to Aon daily, and Aon would fund the Divested Business as needed. There were no

Restructuring

Restructuring	12 Months Ended
	Dec. 31, 2019
Restructuring and Related Activities [Abstract]
Restructuring	Restructuring In 2017, Aon initiated the Restructuring Plan in connection with the sale of the Divested Business. The Restructuring Plan was intended to streamline operations across the organization and deliver greater efficiency, insight, and connectivity. The Company has incurred all remaining costs for the Restructuring Plan and the plan was closed in the fourth quarter of 2019. The Restructuring Plan resulted in cumulative charges of $1,433 million , consisting of $619 million in workforce reduction, $119 million in technology rationalization costs, $69 million in lease consolidation costs, $53 million in non-cash asset impairments, and $573 million in other costs, including certain separation costs associated with the sale of the Divested Business. These charges are included in Compensation and benefits, Information technology, Premises, Depreciation of fixed assets, and Other general expense in the accompanying Consolidated Statements of Income. The Company eliminated 5,832 positions under the Restructuring Program. The following table summarizes restructuring and related expenses by type that were incurred through the end of the Restructuring Plan (in millions): 2019 2018 2017 Completed Plan Total Workforce reduction $ 205 $ 115 $ 299 $ 619 Technology rationalization (1) 39 47 33 119 Lease consolidation (1) 33 28 8 69 Asset impairments 14 13 26 53 Other costs associated with restructuring and separation (1) (2) 160 282 131 573 Total restructuring and related expenses $ 451 $ 485 $ 497 $ 1,433 (1) Total contract termination costs incurred under the Restructuring Plan associated with technology rationalizations, lease consolidations, and other costs associated with restructuring and separation for the twelve months ended December 31, 2019 were $0 million , $33 million , and $13 million , respectively; for the twelve months ended December 31, 2018 were $5 million , $25 million , and $85 million , respectively; and for the twelve months ended December 31, 2017 were $1 million , $8 million , and $3 million , respectively. (2) Other costs associated with the Restructuring Plan include those to separate the Divested Business, as well as moving costs, and consulting and legal fees. These costs are typically recognized when incurred. The changes in the Company’s liabilities for the Restructuring Plan as of December 31, 2019 are as follows (in millions): Restructuring Plan Balance at December 31, 2018 $ 201 Expensed 418 Cash payments (415 ) Foreign currency translation and other — Balance at December 31, 2019 $ 204

Acquisitions and Dispositions o

Acquisitions and Dispositions of Businesses	12 Months Ended
	Dec. 31, 2019
Business Combinations and Discontinued Operations and Disposal Groups [Abstract]
Acquisitions and Dispositions of Businesses	Acquisitions and Dispositions of Businesses Completed Acquisitions The Company completed three acquisitions during the year ended December 31, 2019 and eight acquisitions during the year ended December 31, 2018 . The following table includes the preliminary fair values of consideration transferred, assets acquired, and liabilities assumed as a result of the Company’s acquisitions (in millions): For the year ended December 31, 2019 Consideration transferred Cash $ 42 Deferred and contingent consideration 8 Aggregate consideration transferred $ 50 Assets acquired Cash and cash equivalents $ 4 Receivables, net — Goodwill 34 Intangible assets, net 22 Fixed assets, net 1 Other assets 13 Total assets acquired 74 Liabilities assumed Current liabilities 18 Other non-current liabilities 6 Total liabilities assumed 24 Net assets acquired $ 50 Intangible assets are primarily customer-related and contract-based assets. Those intangible assets acquired as part of a business acquisition in 2019 had a weighted average useful economic life of 7 years . Acquisition related costs incurred and recognized within Other general expense for the year ended December 31, 2019 were $ 1 million . Total revenue for these acquisitions included in the Company’s Consolidated Statement of Income for the year ended December 31, 2019 was approximately $7 million . The results of operations of these acquisitions are included in the Consolidated Financial Statements as of the respective acquisition dates. The Company’s results of operations would not have been materially different if these acquisitions had been reported from the beginning of the period in which they were acquired. 2019 Acquisitions On July 31, 2019, the Company completed the transaction to acquire Ovatio Courtage SAS, an insurance broker based in France. On July 31, 2019, the Company completed the transaction to acquire Zalba-Caldu Correduria de Seguros, S.A., a Spanish insurance broker. On January 1, 2019, the Company completed the transaction to acquire Chapka Assurances SAS based in France. 2018 Acquisitions On December 31, 2018, the Company completed the transaction to acquire certain assets of Bill Beatty Insurance Agency, Inc. based in the United States. On November 15, 2018, the Company completed the transaction to acquire certain business and assets of North Harbour Insurance Services (1985) Limited, a New Zealand-based firm. On October 25, 2018, the Company completed the transaction to acquire 100% capital of GEFASS S.R.L and GE.F.IT S.R.L., Italy-based firms specialized in Bancassurance schemes. On May 9, 2018, the Company completed the transaction to acquire certain assets of 601West, a division of Lee & Hayes, P.L.L.C. based in the United States. On April 24, 2018, the Company completed the transaction to acquire Inspiring Benefits, S.L., a Spain-based firm specialized in employee loyalty, wellbeing, and rewards programs. On March 1, 2018, the Company completed the transaction to acquire the business and assets of the trade credit business of Niche International Business Proprietary Limited, a trade credit brokerage based in Johannesburg, South Africa. On March 1, 2018, the Company completed the transaction to acquire Affinity Risk Partners (Brokers) Pty. Ltd., an insurance broker in Victoria, Australia. On January 19, 2018, the Company completed the transaction to acquire substantially all of the assets of The Burchfield Group, a provider in pharmacy benefit consulting, auditing, and health plan compliance services based in the United States. Completed Dispositions The Company completed eight dispositions during the year ended December 31, 2019 . The Company completed four dispositions during the year ended December 31, 2018 and nine dispositions during the year ended December 31, 2017 , excluding the sale of the Divested Business. Total pretax gains, net of losses, for the year ended December 31, 2019 was $13 million . Total pretax losses, net of gains, for the years ended December 31, 2018 , and 2017 , were $6 million and $16 million , respectively. Gains and losses recognized as a result of a disposition are included in Other income (expense) in the Consolidated Statements of Income. During 2018, the Company recorded a non-cash impairment charge of $176 million related to certain assets and liabilities held for sale. The impairment charge was recognized in Amortization and impairment of intangible assets on the Consolidated Statement of Income. During 2017, the Company recorded a non-cash impairment charge of $380 million to its tradenames associated with the Divested Business as these assets were not sold to the Buyer.

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	12 Months Ended
	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets The changes in the net carrying amount of goodwill for the years ended December 31, 2019 and 2018 , respectively, are as follows (in millions): Balance as of January 1, 2018 $ 8,358 Goodwill related to current year acquisitions 38 Goodwill related to disposals (2 ) Goodwill related to prior year acquisitions 4 Foreign currency translation (227 ) Balance as of December 31, 2018 $ 8,171 Goodwill related to current year acquisitions 34 Goodwill related to disposals (11 ) Goodwill related to prior year acquisitions 2 Foreign currency translation (31 ) Balance as of December 31, 2019 $ 8,165 Other intangible assets by asset class are as follows (in millions): As of December 31 2019 2018 Gross Carrying Amount Accumulated Amortization and Impairment Net Carrying Amount Gross Carrying Amount Accumulated Amortization and Impairment Net Carrying Amount Customer-related and contract-based $ 2,264 $ 1,600 $ 664 $ 2,240 $ 1,444 $ 796 Tradenames 1,029 956 73 1,027 740 287 Technology and other 380 334 46 391 325 66 Total $ 3,673 $ 2,890 $ 783 $ 3,658 $ 2,509 $ 1,149 Amortization expense and impairment charges from finite lived intangible assets were $392 million , $593 million , and $704 million for the years ended December 31, 2019 , 2018 , and 2017 , respectively. The estimated future amortization for finite-lived intangible assets as of December 31, 2019 is as follows (in millions): Estimated Future Amortization For the years ended 2020 $ 223 2021 127 2022 86 2023 76 2024 59 Thereafter 212 Total $ 783

Debt

Debt	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Debt	Debt The following is a summary of outstanding debt (in millions): As of December 31 2019 2018 Commercial paper $ 112 $ 250 5.00% Senior Notes due September 2020 600 599 2.80% Senior Notes due March 2021 399 398 2.20% Senior Notes due November 2022 497 — 4.00% Senior Notes due November 2023 348 348 3.50% Senior Notes due June 2024 597 596 3.875% Senior Notes due December 2025 746 746 2.875% Senior Notes due May 2026 (EUR 500M) 550 562 8.205% Junior Subordinated Notes due January 2027 521 521 4.50% Senior Notes due December 2028 346 347 3.75% Senior Notes due May 2029 744 — 6.25% Senior Notes due September 2040 296 296 4.25% Senior Notes due December 2042 199 198 4.45% Senior Notes due May 2043 246 246 4.60% Senior Notes due June 2044 544 544 4.75% Senior Notes due May 2045 593 592 Other 1 1 Total debt 7,339 6,244 Less: Short-term debt and current portion of long-term debt 712 251 Total long-term debt $ 6,627 $ 5,993 Notes On November 15, 2019, Aon Corporation issued $500 million 2.20% Senior Notes due November 2022. The Company used the net proceeds of the offering to pay down a portion of outstanding commercial paper and for general corporate purposes. In September 2019, the Company’s $600 million 5.00% Senior Notes due September 2020 were classified as Short-term debt and current portion of long-term debt in the Consolidated Statement of Financial Position as the date of maturity is in less than one year. On May 2, 2019, Aon Corporation issued $750 million 3.75% Senior Notes due May 2029. The Company used the net proceeds of the offering to pay down a portion of outstanding commercial paper and for general corporate purposes. On December 3, 2018, Aon Corporation issued $350 million 4.50% Senior Notes due December 2028. The Company used the net proceeds of the offering to pay down a portion of outstanding commercial paper and for general corporate purposes. Each of the notes issued by Aon plc is fully and unconditionally guaranteed by Aon Corporation, and each of the notes issued by Aon Corporation is fully and unconditionally guaranteed by Aon plc. Refer to Note 19 “Guarantee of Registered Securities” for additional information regarding guarantees of outstanding debt securities. Each of the notes described and identified in the table above contains customary representations, warranties, and covenants, and the Company was in compliance with all such covenants as of December 31, 2019 . Repayments of total debt are as follows (in millions): 2020 $ 712 2021 400 2022 500 2023 350 2024 600 Thereafter 4,882 Total Repayments 7,444 Unamortized discounts, premiums, and debt issuance costs (105 ) Total Debt $ 7,339 Revolving Credit Facilities As of December 31, 2019 , Aon plc had two primary committed credit facilities outstanding: its $900 million multi-currency U.S. credit facility expiring in February 2022 and its $400 million multi-currency U.S. credit facility expiring in October 2023. Each of these facilities includes customary representations, warranties, and covenants, including financial covenants that require Aon to maintain specified ratios of adjusted consolidated EBITDA to consolidated interest expense and consolidated debt to adjusted consolidated EBITDA, in each case, tested quarterly. At December 31, 2019 , Aon did not have borrowings under either credit facility, and was in compliance with the financial covenants and all other covenants contained therein during the twelve months ended December 31, 2019 . Commercial Paper Aon Corporation, a wholly owned subsidiary of Aon plc, has established a U.S. commercial paper program, and Aon plc has established a European multi-currency commercial paper program. Commercial paper may be issued in aggregate principal amounts of up to $600 million under the U.S. program and €525 million under the European program, not to exceed the amount of the Company’s committed credit, which was $1.3 billion at December 31, 2019 . The U.S. commercial paper program is fully and unconditionally guaranteed by Aon plc and the European multi-currency commercial paper program is fully and unconditionally guaranteed by Aon Corporation. Commercial paper outstanding, which is included in Short-term debt and current portion of long-term debt in the Company’s Consolidated Statements of Financial Position, is as follows (in millions): As of December 31 2019 2018 Commercial paper outstanding $ 112 $ 250 The weighted average commercial paper outstanding and its related interest rates are as follows (in millions, except percentages): Years ended December 31 2019 2018 Weighted average commercial paper outstanding $ 511 $ 580 Weighted average interest rate of commercial paper outstanding 0.27 % 0.84 %

Lease Commitments

Lease Commitments	12 Months Ended
	Dec. 31, 2019
Leases, Operating [Abstract]
Lease Commitments	Lease Commitments The classification of operating and finance lease asset and liability balances within the Consolidated Statement of Financial Position is as follows (in millions): As of December 31, 2019 Assets Operating lease assets Operating lease right-of-use assets $ 929 Finance lease assets Other non-current assets 100 Total lease assets $ 1,029 Liabilities Current lease liabilities Operating Other current liabilities $ 186 Finance Other current liabilities 24 Non-current lease liabilities Operating Non-current operating lease liabilities 944 Finance Other non-current liabilities 76 Total lease liabilities $ 1,230 The components of lease costs are as follows (in millions): Year Ended December 31, 2019 Operating lease cost $ 234 Finance lease costs Amortization of leased assets 26 Interest on lease liabilities 2 Variable lease cost 60 Short-term lease cost (1) 5 Sublease income (32 ) Net lease cost $ 295 (1) Short-term lease cost does not include expenses related to leases with a lease term of one month or less. Weighted average remaining lease term and discount rate related to operating and finance leases are as follows: As of December 31, 2019 Weighted average remaining lease term (years) Operating leases 7.9 Finance leases 4.4 Weighted average discount rate Operating leases 3.2 % Finance leases 2.0 % Other cash and non-cash related activities are as follows (in millions): Year Ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows for operating leases $ 264 Financing cash flows for finance leases $ 17 Non-cash related activities ROU assets obtained in exchange for new operating lease liabilities $ 155 ROU assets obtained in exchange for new finance lease liabilities $ 48 Operating lease ROU asset expense (1) $ 195 Changes in Non-current operating lease liabilities (1) $ (70 ) (1) The Company has recorded non-cash changes in Operating lease ROU assets and Non-current operating lease liabilities through Other assets and liabilities in Cash flows from operations within the Consolidated Statement of Cash Flows. Maturity analysis of operating and finance leases as of December 31, 2019 are as follows (in millions): Operating Leases Finance Leases Total 2020 $ 208 $ 26 $ 234 2021 202 26 228 2022 177 20 197 2023 141 18 159 2024 113 15 128 Thereafter 432 — 432 Total undiscounted future minimum lease payments 1,273 105 1,378 Less: Imputed interest (143 ) (5 ) (148 ) Present value of lease liabilities $ 1,130 $ 100 $ 1,230 At December 31, 2018, future minimum rental payments required under operating leases that have initial or remaining non-cancelable lease terms in excess of one year are as follows (in millions): Gross rental commitments Rentals from subleases Net rental commitments 2019 $ 303 $ (34 ) $ 269 2020 253 (30 ) 223 2021 221 (30 ) 191 2022 182 (30 ) 152 2023 148 (12 ) 136 Thereafter 472 (5 ) 467 Total minimum payments required $ 1,579 $ (141 ) $ 1,438
Lease Commitments	Lease Commitments The classification of operating and finance lease asset and liability balances within the Consolidated Statement of Financial Position is as follows (in millions): As of December 31, 2019 Assets Operating lease assets Operating lease right-of-use assets $ 929 Finance lease assets Other non-current assets 100 Total lease assets $ 1,029 Liabilities Current lease liabilities Operating Other current liabilities $ 186 Finance Other current liabilities 24 Non-current lease liabilities Operating Non-current operating lease liabilities 944 Finance Other non-current liabilities 76 Total lease liabilities $ 1,230 The components of lease costs are as follows (in millions): Year Ended December 31, 2019 Operating lease cost $ 234 Finance lease costs Amortization of leased assets 26 Interest on lease liabilities 2 Variable lease cost 60 Short-term lease cost (1) 5 Sublease income (32 ) Net lease cost $ 295 (1) Short-term lease cost does not include expenses related to leases with a lease term of one month or less. Weighted average remaining lease term and discount rate related to operating and finance leases are as follows: As of December 31, 2019 Weighted average remaining lease term (years) Operating leases 7.9 Finance leases 4.4 Weighted average discount rate Operating leases 3.2 % Finance leases 2.0 % Other cash and non-cash related activities are as follows (in millions): Year Ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows for operating leases $ 264 Financing cash flows for finance leases $ 17 Non-cash related activities ROU assets obtained in exchange for new operating lease liabilities $ 155 ROU assets obtained in exchange for new finance lease liabilities $ 48 Operating lease ROU asset expense (1) $ 195 Changes in Non-current operating lease liabilities (1) $ (70 ) (1) The Company has recorded non-cash changes in Operating lease ROU assets and Non-current operating lease liabilities through Other assets and liabilities in Cash flows from operations within the Consolidated Statement of Cash Flows. Maturity analysis of operating and finance leases as of December 31, 2019 are as follows (in millions): Operating Leases Finance Leases Total 2020 $ 208 $ 26 $ 234 2021 202 26 228 2022 177 20 197 2023 141 18 159 2024 113 15 128 Thereafter 432 — 432 Total undiscounted future minimum lease payments 1,273 105 1,378 Less: Imputed interest (143 ) (5 ) (148 ) Present value of lease liabilities $ 1,130 $ 100 $ 1,230 At December 31, 2018, future minimum rental payments required under operating leases that have initial or remaining non-cancelable lease terms in excess of one year are as follows (in millions): Gross rental commitments Rentals from subleases Net rental commitments 2019 $ 303 $ (34 ) $ 269 2020 253 (30 ) 223 2021 221 (30 ) 191 2022 182 (30 ) 152 2023 148 (12 ) 136 Thereafter 472 (5 ) 467 Total minimum payments required $ 1,579 $ (141 ) $ 1,438

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income before income tax from continuing operations and the provision for income tax from continuing operations consist of the following (in millions): Years ended December 31 2019 2018 2017 Income before income taxes: U.K. $ 228 $ (240 ) $ (420 ) U.S. (219 ) (601 ) (765 ) Other 1,862 2,087 1,870 Total $ 1,871 $ 1,246 $ 685 Income tax expense (benefit): Current: U.K. $ 20 $ 21 $ 1 U.S. federal 22 101 48 U.S. state and local 41 35 18 Other 250 214 201 Total current tax expense $ 333 $ 371 $ 268 Deferred tax expense (benefit): U.K. $ 35 $ 19 $ (5 ) U.S. federal (20 ) (165 ) 12 U.S. state and local (27 ) (56 ) (35 ) Other (24 ) (23 ) 10 Total deferred tax benefit $ (36 ) $ (225 ) $ (18 ) Total income tax expense $ 297 $ 146 $ 250 Income before income taxes shown above is based on the location of the business unit to which such earnings are attributable for tax purposes. In addition, because the earnings shown above may, in some cases, be subject to taxation in more than one country, the income tax provision shown above as U.K., U.S. or Other may not correspond to the geographic attribution of the earnings. The Company performs a reconciliation of the income tax provisions based on its domicile and statutory rate at each reporting period. The 2019 , 2018 , and 2017 reconciliations are based on the U.K. statutory corporate tax rate of 19.0% , 19.0% , and 19.3% , respectively. The reconciliation to the provisions from continuing operations reflected in the Consolidated Financial Statements is as follows: Years ended December 31 2019 2018 2017 Statutory tax rate 19.0% 19.0% 19.3% U.S. state income taxes, net of U.S. federal benefit 0.5 (0.4) (1.5) Taxes on international operations (1) (6.0) (7.3) (30.3) Nondeductible expenses 1.6 2.7 3.4 Adjustments to prior year tax requirements 0.1 0.9 2.0 Adjustments to valuation allowances 1.8 3.8 (1.8) Change in uncertain tax positions 2.2 0.9 1.6 Excess tax benefits related to shared based compensation (2) (2.8) (3.6) (8.0) U.S. Tax Reform impact (3) (0.3) 7.1 51.2 Loss on disposition — (10.2) — Other — net (0.2) (1.2) 0.6 Effective tax rate 15.9% 11.7% 36.5% (1) The Company determines the adjustment for taxes on international operations based on the difference between the statutory tax rate applicable to earnings in each foreign jurisdiction and the enacted rate of 19.0% , 19.0% and 19.3% at December 31, 2019 , 2018 , and 2017 , respectively. The benefit to the Company’s effective income tax rate from taxes on international operations relates to benefits from lower-taxed global operations, primarily due to the use of global funding structures and the tax holiday in Singapore. The impact decreased from 2017 to 2018 primarily as a result of the decrease in the U.S. federal tax rate. (2) With the adoption of ASU 2016-09 in 2017, excess tax benefits and deficiencies from share-based payment transactions are recognized as income tax expense or benefit in the Company’s Consolidated Statements of Income. (3) The impact of the Tax Reform Act including the Transition Tax, the re-measurement of U.S. deferred tax assets and liabilities from 35% to 21%, withholding tax accruals, and the allocation of tax benefit between continuing operations and discontinued operations related to utilization of foreign tax credits. For the tax impact of discontinued operations, see Note 5 “Discontinued Operations”. The Company has elected to account for global intangible low-taxed income (“GILTI”) in the period in which it is incurred, and therefore has not provided deferred tax impacts of GILTI in its Consolidated Financial Statements. The components of the Company’s deferred tax assets and liabilities are as follows (in millions): As of December 31 2019 2018 Deferred tax assets: Net operating loss, capital loss, interest, and tax credit carryforwards $ 440 $ 563 Employee benefit plans 373 351 Lease liabilities 247 — Accrued interest 116 — Other accrued expenses 68 98 Investment basis differences 28 28 Deferred revenue 24 29 Lease and service guarantees 3 5 Other 57 46 Total 1,356 1,120 Valuation allowance on deferred tax assets (200 ) (171 ) Total $ 1,156 $ 949 Deferred tax liabilities: Intangibles and property, plant and equipment $ (251 ) $ (310 ) Lease right-of-use asset (219 ) — Deferred costs (128 ) (143 ) Unremitted earnings (28 ) (30 ) Unrealized foreign exchange gains (26 ) (26 ) Other accrued expenses (25 ) (36 ) Other (33 ) (24 ) Total $ (710 ) $ (569 ) Net deferred tax asset $ 446 $ 380 Deferred income taxes (assets and liabilities have been netted by jurisdiction) have been classified in the Consolidated Statements of Financial Position as follows (in millions): As of December 31 2019 2018 Deferred tax assets — non-current $ 645 $ 561 Deferred tax liabilities — non-current (199 ) (181 ) Net deferred tax asset $ 446 $ 380 In assessing the realizability of deferred tax assets, the Company considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized and adjusts the valuation allowance accordingly. Considerations with respect to the realizability of deferred tax assets include the period of expiration of the deferred tax asset, historical earnings and projected future taxable income by jurisdiction as well as tax liabilities for the tax jurisdiction to which the tax asset relates. Significant management judgment is required in determining the assumptions and estimates related to the amount and timing of future taxable income. Valuation allowances have been established primarily with regard to the tax benefits of certain net operating loss, capital loss, and interest carryforwards. Valuation allowances increased by $29 million as of December 31, 2019 , when compared to December 31, 2018 . The change is primarily attributable to interest carryforwards generated in 2019 for which utilization is subject to limitation. The Company generally intends to limit distributions from foreign subsidiaries to earnings previously taxed in the U.S., primarily as a result of the Transition Tax or GILTI. As of December 31, 2019 , the Company has accrued $28 million for local country income taxes, withholding taxes and state income taxes on those undistributed earnings that are not indefinitely reinvested. The Company has not provided for deferred taxes on outside basis differences in our investments in our foreign subsidiaries that are unrelated to these accumulated undistributed earnings, as these outside basis differences are indefinitely reinvested. A determination of the unrecognized deferred taxes related to these other components of our outside basis differences is not practicable. The Company had the following net operating loss, capital loss, and interest carryforwards (in millions): As of December 31 2019 2018 U.K. Operating loss carryforwards $ 438 $ 541 Capital loss carryforwards $ 411 $ 400 Interest carryforwards $ 203 $ 53 U.S. Federal operating loss carryforwards $ 1 $ 2 Federal capital loss carryforwards $ 112 $ 367 Federal interest carryforwards $ 355 $ 424 State operating loss carryforwards $ 376 $ 315 State capital loss carryforwards $ 123 $ 221 State interest carryforwards $ 172 $ 227 Other Non-U.S. Operating loss carryforwards $ 308 $ 369 Capital loss carryforwards $ 29 $ 30 Interest carryforwards $ 159 $ 186 The U.K. operating losses, capital losses, and interest carryforward each have an indefinite carryforward period. The federal operating loss carryforwards generated through December 31, 2017 expire at various dates between 2034 and 2036 while federal operating loss carryforwards generated after this date have indefinite carryforward periods. State net operating losses as of December 31, 2019 have various carryforward periods and will begin to expire in 2020. Federal and state capital losses can be carried forward until 2023. Federal and state interest carryforwards have indefinite carryforward periods. Operating and capital losses in other non-U.S. jurisdictions have various carryforward periods and will begin to expire in 2020. The interest carryforwards in other non-U.S. jurisdictions have indefinite carryforward periods. During 2012, the Company was granted a tax holiday for the period from October 1, 2012 through September 30, 2022, with respect to withholding taxes and certain income derived from services in Singapore. This tax holiday and reduced withholding tax rate may be extended when certain conditions are met or may be terminated early if certain conditions are not met. The benefit realized was approximately $90 million , $77 million , and $45 million during the years ended December 31, 2019 , 2018 , and 2017 , respectively. The impact of this tax holiday on diluted earnings per share was $0.37 , $0.31 , and $0.17 during the years ended December 31, 2019 , 2018 , and 2017 , respectively. Uncertain Tax Positions The following is a reconciliation of the Company’s beginning and ending amount of uncertain tax positions (in millions): 2019 2018 Balance at January 1 $ 279 $ 280 Additions based on tax positions related to the current year 23 18 Additions for tax positions of prior years 12 10 Reductions for tax positions of prior years (5 ) (24 ) Settlements (5 ) — Business combinations — 1 Lapse of statute of limitations (5 ) (6 ) Foreign currency translation — — Balance at December 31 $ 299 $ 279 The Company’s liability for uncertain tax positions as of December 31, 2019 , 2018 , and 2017 , includes $248 million , $228 million , and $219 million , respectively, related to amounts that would impact the effective tax rate if recognized. It is possible that the amount of unrecognized tax benefits may change in the next twelve months; however, the Company does not expect the change to have a significant impact on its consolidated statements of income or consolidated balance sheets. These changes may be the result of settlements of ongoing audits. At this time, an estimate of the range of the reasonably possible outcomes within the twelve months cannot be made. The Company recognizes interest and penalties related to uncertain tax positions in its provision for income taxes. The Company accrued potential interest and penalties of $24 million , $22 million , and $11 million in 2019 , 2018 , and 2017 , respectively. The Company recorded a liability for interest and penalties of $99 million , $77 million , and $55 million as of December 31, 2019 , 2018 , and 2017 , respectively. The Company and its subsidiaries file income tax returns in their respective jurisdictions. The Company has substantially concluded all U.S. federal income tax matters for years through 2007. Material U.S. state and local income tax jurisdiction examinations have been concluded for years through 2005. The Company has concluded income tax examinations in its primary non-U.S. jurisdictions through 2008.

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Shareholders' Equity	Shareholders’ Equity Distributable Reserves As a company incorporated in England and Wales, Aon is required under U.K. law to have available “distributable reserves” to make share repurchases or pay dividends to shareholders. Distributable reserves are created through the earnings of the U.K. parent company and, among other methods, through a reduction in share capital approved by the High Court of Justice in England. Distributable reserves are not directly linked to a U.S. GAAP reported amount (e.g., retained earnings). As of December 31, 2019 and 2018 , the Company had distributable reserves in excess of $32.4 billion and $2.2 billion , respectively. On July 16, 2019, we completed a reduction in share capital to create additional distributable reserves of $31 billion to support the payment of possible future dividends or future share repurchases, if and to the extent declared by the directors in compliance with their duties under U.K. law. Ordinary Shares Aon has a share repurchase program authorized by the Company’s Board of Directors. The Repurchase Program was established in April 2012 with $5.0 billion in authorized repurchases, and was increased by $5.0 billion in authorized repurchases in each of November 2014 and June 2017 for a total of $15.0 billion in repurchase authorizations. Under the Repurchase Program, Class A Ordinary Shares may be repurchased through the open market or in privately negotiated transactions, from time to time, based on prevailing market conditions, and will be funded from available capital. The following table summarizes the Company’s share repurchase activity (in millions, except per share data): Twelve months ended December 31 2019 2018 Shares repurchased 10.5 10.0 Average price per share $ 186.33 $ 143.94 Costs recorded to retained earnings Total repurchase cost $ 1,950 $ 1,447 Additional associated costs 10 7 Total costs recorded to retained earnings $ 1,960 $ 1,454 At December 31, 2019 , the remaining authorized amount for share repurchase under the Repurchase Program was approximately $2.0 billion . Under the Repurchase Program, the Company has repurchased a total of 128.7 million shares for an aggregate cost of approximately $13.0 billion . Net Income Per Share Weighted average ordinary shares outstanding are as follows (in millions): Years ended December 31 2019 2018 2017 Basic weighted average ordinary shares outstanding 238.6 245.2 258.5 Dilutive effect of potentially issuable shares 2.0 1.8 2.2 Diluted weighted average ordinary shares outstanding 240.6 247.0 260.7 Potentially issuable shares are not included in the computation of diluted net income per share if their inclusion would be antidilutive. There were no shares excluded from the calculation in any of the years presented. Dividends During 2019 , 2018 , and 2017 , the Company paid dividends of $410 million , $382 million , and $364 million , respectively, to holders of its Class A Ordinary Shares. Dividends paid per Class A Ordinary Share were $1.72 , $1.56 , and $1.41 for the years ended December 31, 2019 , 2018 , and 2017 , respectively. Accumulated Other Comprehensive Loss Changes in Accumulated other comprehensive loss by component, net of related tax, are as follows (in millions): Change in Fair Value of Financial Instruments (1) Foreign Currency Translation Adjustments Postretirement Benefit Obligation (2) Total Balance at January 1, 2017 $ (37 ) $ (1,264 ) $ (2,611 ) $ (3,912 ) Other comprehensive income (loss) before reclassifications Other comprehensive income (loss) before reclassifications 18 397 (220 ) 195 Tax benefit (expense) (3 ) (5 ) 55 47 Other comprehensive income (loss) before reclassifications, net 15 392 (165 ) 242 Amounts reclassified from accumulated other comprehensive income (loss) Amounts reclassified from accumulated other comprehensive income (loss) (2 ) (7 ) 236 227 Tax benefit (expense) (1 ) — (52 ) (53 ) Amounts reclassified from accumulated other comprehensive income (loss), net (3 ) (7 ) 184 174 Net current period other comprehensive income (loss) 12 385 19 416 Balance at December 31, 2017 (25 ) (879 ) (2,592 ) (3,496 ) Adoption of new accounting guidance (3) (1 ) — — (1 ) Balance at January 1, 2018 (26 ) (879 ) (2,592 ) (3,497 ) Other comprehensive income (loss) before reclassifications Other comprehensive income (loss) before reclassifications (15 ) (437 ) (124 ) (576 ) Tax benefit (expense) 18 (3 ) 28 43 Other comprehensive income (loss) before reclassifications, net 3 (440 ) (96 ) (533 ) Amounts reclassified from accumulated other comprehensive income (loss) Amounts reclassified from accumulated other comprehensive income (loss) 11 — 146 157 Tax benefit (expense) (3 ) — (33 ) (36 ) Amounts reclassified from accumulated other comprehensive income (loss), net 8 — 113 121 Net current period other comprehensive income (loss) 11 (440 ) 17 (412 ) Balance at December 31, 2018 (15 ) (1,319 ) (2,575 ) (3,909 ) Other comprehensive income (loss) before reclassifications Other comprehensive income (loss) before reclassifications (10 ) 15 (287 ) (282 ) Tax benefit (expense) 1 (1 ) 58 58 Other comprehensive income (loss) before reclassifications, net (9 ) 14 (229 ) (224 ) Amounts reclassified from accumulated other comprehensive income (loss) Amounts reclassified from accumulated other comprehensive income (loss) 14 — 109 123 Tax benefit (expense) (2 ) — (21 ) (23 ) Amounts reclassified from accumulated other comprehensive income (loss), net 12 — 88 100 Net current period other comprehensive income (loss) 3 14 (141 ) (124 ) Balance at December 31, 2019 $ (12 ) $ (1,305 ) $ (2,716 ) $ (4,033 ) (1) Reclassifications from this category included in Accumulated other comprehensive loss are recorded in Revenue, Interest expense, and Compensation and benefits in the accompanying Consolidated Statements of Income. See Note 15 “Derivatives and Hedging” for additional information regarding the Company’s derivative and hedging activity. (2) Reclassifications from this category included in Accumulated other comprehensive loss are recorded in Other income (expense). (3) Refer to Note 2 “Summary of Significant Accounting Principles and Practices ” for further information.

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Employee Benefits	Employee Benefits Defined Contribution Savings Plans Aon maintains defined contribution savings plans for the benefit of its employees. The expense recognized for these plans is included in Compensation and benefits in the Consolidated Statements of Income. The expense for the significant plans in the U.S., U.K., Netherlands and Canada is as follows (in millions): Years ended December 31 2019 2018 2017 U.S. $ 98 $ 98 $ 105 U.K. 41 45 43 Netherlands and Canada 25 25 25 Total $ 164 $ 168 $ 173 Pension and Other Postretirement Benefits The Company sponsors defined benefit pension and postretirement health and welfare plans that provide retirement, medical, and life insurance benefits. The postretirement health care plans are contributory, with retiree contributions adjusted annually, and the life insurance and pension plans are generally noncontributory. The significant U.S., U.K., Netherlands and Canada pension plans are closed to new entrants. Pension Plans The following tables provide a reconciliation of the changes in the projected benefit obligations and fair value of assets for the years ended December 31, 2019 and 2018 , and a statement of the funded status as of December 31, 2019 and 2018 , for Aon’s significant U.K., U.S., and other major pension plans, which are located in the Netherlands and Canada. These plans represent approximately 91% of the Company’s projected benefit obligations. U.K. U.S. Other (millions) 2019 2018 2019 2018 2019 2018 Change in projected benefit obligation At January 1 $ 4,129 $ 4,893 $ 2,877 $ 3,155 $ 1,271 $ 1,401 Service cost — — — — — — Interest cost 109 109 108 99 27 27 Plan amendment 10 13 — — — — Settlements (22 ) (176 ) — — — — Actuarial loss (gain) 594 (297 ) 373 (221 ) 177 (47 ) Benefit payments (168 ) (160 ) (166 ) (156 ) (42 ) (43 ) Foreign currency impact 127 (253 ) — — (8 ) (67 ) As of December 31 $ 4,779 $ 4,129 $ 3,192 $ 2,877 $ 1,425 $ 1,271 Accumulated benefit obligation at end of year $ 4,779 $ 4,129 $ 3,192 $ 2,877 $ 1,391 $ 1,247 Change in fair value of plan assets At January 1 $ 5,225 $ 5,906 $ 1,796 $ 1,958 $ 1,155 $ 1,256 Actual return on plan assets 687 (125 ) 398 (141 ) 182 (19 ) Employer contributions 78 97 38 135 19 20 Settlements (22 ) (176 ) — — — — Benefit payments (168 ) (160 ) (166 ) (156 ) (42 ) (43 ) Foreign currency impact 159 (317 ) — — (11 ) (59 ) As of December 31 $ 5,959 $ 5,225 $ 2,066 $ 1,796 $ 1,303 $ 1,155 Market related value at end of year $ 5,959 $ 5,225 $ 1,969 $ 1,981 $ 1,303 $ 1,155 Amount recognized in Statement of Financial Position as of December 31 Funded status $ 1,180 $ 1,096 $ (1,126 ) $ (1,081 ) $ (122 ) $ (116 ) Unrecognized prior-service cost 40 30 1 3 (6 ) (7 ) Unrecognized loss 1,204 1,106 1,762 1,705 460 440 Net amount recognized $ 2,424 $ 2,232 $ 637 $ 627 $ 332 $ 317 In September 2018, the Company made a cash contribution of $100 million to the qualified U.S. pension plan, which allowed the pension contribution tax deduction to be taken at the 2017 federal tax rate of 35%. Amounts recognized in the Consolidated Statements of Financial Position consist of (in millions): U.K. U.S. Other 2019 2018 2019 2018 2019 2018 Prepaid benefit cost (1) $ 1,200 $ 1,113 $ — $ — $ — $ — Accrued benefit liability - current (2) (1 ) (1 ) (50 ) (46 ) (5 ) (5 ) Accrued benefit liability - non-current (3) (19 ) (16 ) (1,076 ) (1,035 ) (117 ) (111 ) Accumulated other comprehensive loss 1,244 1,136 1,763 1,708 454 433 Net amount recognized $ 2,424 $ 2,232 $ 637 $ 627 $ 332 $ 317 (1) Included in Prepaid pension (2) Included in Other current liabilities (3) Included in Pension, other postretirement, and postemployment liabilities Amounts recognized in Accumulated other comprehensive loss (income) that have not yet been recognized as components of net periodic benefit cost at December 31, 2019 and 2018 consist of (in millions): U.K. U.S. Other 2019 2018 2019 2018 2019 2018 Net loss $ 1,204 $ 1,106 $ 1,762 $ 1,705 $ 460 $ 440 Prior service cost (income) 40 30 1 3 (6 ) (7 ) Total $ 1,244 $ 1,136 $ 1,763 $ 1,708 $ 454 $ 433 In 2019 , U.S. plans with a projected benefit obligation (“PBO”) and an accumulated benefit obligation (“ABO”) in excess of the fair value of plan assets had a PBO of $3.1 billion , an ABO of $3.1 billion , and plan assets with a fair value of $2.0 billion . U.K. plans with a PBO and an ABO in excess of the fair value of plan assets had a PBO of $54 million , an ABO of $54 million and plan assets with a fair value of $34 million . Other plans with a PBO in excess of the fair value of plan assets had a PBO of $1.4 billion and plan assets with a fair value of $1.3 billion , and other plans with an ABO in excess of the fair value of plan assets had an ABO of $406 million and plan assets with a fair value of $311 million . In 2018 , U.S. plans with a PBO and an ABO in excess of the fair value of plan assets had a PBO of $2.9 billion , an ABO of $2.9 billion , and plan assets of $1.8 billion . U.K. plans with a PBO in excess of the fair value of plan assets had a PBO of $47 million and plan assets with a fair value of $30 million , and plans with an ABO in excess of the fair value of plan assets had an ABO of $47 million and plan assets with a fair value of $30 million . Other plans with a PBO in excess of the fair value of plan assets had a PBO of $1.2 billion and plan assets with a fair value of $1.1 billion , and plans with an ABO in excess of the fair value of plan assets had an ABO of $363 million and plan assets with a fair value of $271 million . Service cost is reported in Compensation and benefits and all other components are reported in Other income (expense) as follows (in millions): U.K. U.S. Other 2019 2018 2017 2019 2018 2017 2019 2018 2017 Service cost $ — $ — $ — $ — $ — $ — $ — $ — $ — Interest cost 109 109 123 108 99 96 27 27 26 Expected return on plan assets, net of administration expenses (191 ) (192 ) (199 ) (136 ) (144 ) (140 ) (40 ) (45 ) (47 ) Amortization of prior-service cost 1 1 1 2 2 2 — — — Amortization of net actuarial loss 29 28 31 53 59 50 12 12 11 Net periodic benefit (income) cost (52 ) (54 ) (44 ) 27 16 8 (1 ) (6 ) (10 ) Settlement expense 5 37 125 — — — — — — Total net periodic benefit cost (income) $ (47 ) $ (17 ) $ 81 $ 27 $ 16 $ 8 $ (1 ) $ (6 ) $ (10 ) The Company uses a full-yield curve approach in the estimation of the service and interest cost components of net periodic pension and postretirement benefit cost for its major pension and other postretirement benefit plans. This estimation was obtained by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to the relevant projected cash flows. Transfer payments from certain U.K. pension plans exceeded the plan’s service and interest cost. This triggered settlement accounting which required immediate recognition of a portion of the accumulated losses associated with the plan. Consequently, the Company recognized a non-cash settlement charge for approximately £4 million ( $5 million using December 31, 2019 exchange rates). In March 2017, the Company approved a plan to offer a voluntary one-time lump sum payment option to certain eligible employees of the Company’s U.K. pension plans that, if accepted, would settle the Company’s pension obligations to them. The lump sum cash payment offer closed in 2018. As of December 31, 2018, lump sum payments from plan assets of £139 million ( $176 million using December 31, 2018 exchange rates) were paid. As a result of this settlement, the Company remeasured the assets and liabilities of the U.K. pension plan during the fourth quarter of 2018, which in aggregate resulted in a reduction to the projected benefit obligation of £122 million ( $154 million using December 31, 2018 exchange rates) as well as a non-cash settlement charge of £28 million ( $37 million using average exchange rates) for the year ended December 31, 2018. In total for 2017, lump sum payments from plan assets of £371 million ( $496 million using December 31, 2017 exchange rates) were paid. As a result of this settlement, the Company remeasured the assets and liabilities of the U.K. pension plan during the fourth quarter of 2017, which in aggregate resulted in a reduction to the projected benefit obligation of £325 million ( $434 million using December 31, 2017 exchange rates) as well as a non-cash settlement charge of £93 million ( $125 million using average December 31, 2017 exchange rates) in the fourth quarter of 2017. The weighted-average assumptions used to determine benefit obligations are as follows: U.K. U.S. (1) Other 2019 2018 2019 2018 2019 2018 Discount rate 2.09% 2.95% 2.72 - 3.17% 3.92 - 4.26% 0.91 - 3.10% 1.89 - 3.88% Rate of compensation increase 3.24 - 3.74% 3.73 - 4.23% N/A N/A 1.00 - 3.00% 1.00 - 3.00% Underlying price inflation 1.78% 1.88% N/A N/A 2.00% 2.00% (1) U.S. pension plans are frozen and therefore not impacted by compensation increases or price inflation. The weighted-average assumptions used to determine the net periodic benefit cost are as follows: U.K. U.S. Other 2019 2018 2017 2019 2018 2017 2019 2018 2017 Discount rate 2.95% 2.63% 2.77% 3.92 - 4.26% 3.27 - 3.61% 3.53 - 4.11% 1.89 - 3.88% 1.78 - 3.39% 1.85 - 3.81% Expected return on plan assets, net of administration expenses 3.64% 3.34% 3.36% 7.05% 7.71% 7.88% 2.50 - 4.10% 1.70 - 4.85% 2.68 - 5.15% Rate of compensation increase 3.73 - 4.23% 3.70 - 4.20% 3.70 - 4.20% N/A N/A N/A 1.00 - 3.00% 1.00 - 3.00% 1.00 - 3.50% The amounts in Accumulated other comprehensive loss expected to be recognized as components of net periodic benefit cost during 2020 , not including voluntary one-time lump sum payments, are $68 million in the U.S. and $44 million outside the U.S. Expected Return on Plan Assets To determine the expected long-term rate of return on plan assets, the historical performance, investment community forecasts, and current market conditions are analyzed to develop expected returns for each asset class used by the plans. The expected returns for each asset class are weighted by the target allocations of the plans. The expected return of 7.05% on U.S. plan assets reflects a portfolio that is seeking asset growth through a higher equity allocation while maintaining prudent risk levels. The portfolio contains certain assets that have historically resulted in higher returns, as well as other financial instruments to minimize downside risk. No plan assets are expected to be returned to the Company during 2020 . Fair value of plan assets The Company determined the fair value of plan assets through numerous procedures based on the asset class and available information. Refer to Note 16 “Fair Value Measurements and Financial Instruments” for a description of the procedures performed to determine the fair value of the plan assets. The fair values of the Company’s U.S. pension plan assets at December 31, 2019 and December 31, 2018 , by asset category, are as follows (in millions): Fair Value Measurements Using Asset Category Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 77 $ 77 $ — $ — Equity investments: Equity securities 195 195 — — Equity derivatives 22 — 22 — Pooled funds (2) 583 — — — Fixed income investments: Corporate bonds 128 — 128 — Government and agency bonds 199 162 37 — Asset-backed securities 3 — 3 — Pooled funds (2) 545 — — — Other investments: Real estate (2) (3) 133 — — — Alternative investments (2) (4) 181 — — — Total $ 2,066 $ 434 $ 190 $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of property funds and trusts holding direct real estate investments. (4) Consists of limited partnerships, private equity, and hedge funds. Fair Value Measurements Using Asset Category Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 130 $ 130 $ — $ — Equity investments: Equity securities 384 384 — — Equity derivatives (14 ) — (14 ) — Pooled funds (2) 285 — — — Fixed income investments: Corporate bonds 111 — 111 — Government and agency bonds 126 95 31 — Asset-backed securities 2 — 2 — Pooled funds (2) 417 — — — Other investments: Real estate and REITs (3) 78 78 — — Alternative investments (2) (4) 277 — — — Total $ 1,796 $ 687 $ 130 $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of exchange traded real estate investment trusts (“REITs”). (4) Consists of limited partnerships, private equity, and hedge funds. The fair values of the Company’s major U.K. pension plan assets at December 31, 2019 and December 31, 2018 , by asset category, are as follows (in millions): Fair Value Measurements Using Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 81 $ 81 $ — $ — Equity investments: Pooled funds (2) 119 — — — Fixed income investments: Derivatives (3) (1,205 ) — (1,205 ) — Government and agency bonds 2,667 2,667 — — Annuities 1,849 — — 1,849 Pooled funds (2) 1,486 — — — Other investments: Real estate (2) (4) 180 — — — Alternative investments (2) (5) 782 — — — Total $ 5,959 $ 2,748 $ (1,205 ) $ 1,849 (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of equity securities and equity derivatives, including repurchase agreements. (4) Consists of property funds and trusts holding direct real estate investments. (5) Consists of limited partnerships, private equity, and hedge funds. Fair Value Measurements Using Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 96 $ 96 $ — $ — Equity investments: Pooled funds (2) 212 — — — Fixed income investments: Derivatives (3) (949 ) — (949 ) — Corporate bonds 367 — 367 — Government and agency bonds 2,079 2,079 — — Annuities 1,688 — — 1,688 Pooled funds (2) 889 — — — Other investments: Real estate (2) (4) 149 — — — Alternative investments (2) (5) 694 — — — Total $ 5,225 $ 2,175 $ (582 ) $ 1,688 (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of equity securities and equity derivatives, including repurchase agreements. (4) Consists of property funds and trusts holding direct real estate investments. (5) Consists of limited partnerships, private equity, and hedge funds. The following table presents the changes in the Level 3 fair-value category in the Company’s U.K. pension plans for the years ended December 31, 2019 and December 31, 2018 (in millions): Fair Value Measurements Using Level 3 Inputs Annuities Balance at January 1, 2018 $ 1,909 Actual return on plan assets: Relating to assets still held at December 31, 2018 (122 ) Purchases, sales and settlements—net 7 Foreign exchange (106 ) Balance at December 31, 2018 1,688 Actual return on plan assets: Relating to assets still held at December 31, 2019 113 Foreign exchange 48 Balance at December 31, 2019 $ 1,849 The fair values of the Company’s other major pension plan assets at December 31, 2019 and December 31, 2018 , by asset category, are as follows (in millions): Fair Value Measurements Using Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 5 $ 5 $ — $ — Equity investments: Pooled funds (2) 323 — — — Fixed income investments: Pooled funds (2) 907 — — — Other investments: Alternative investments (2) (3) 62 — — — Real estate (2) (4) 6 — — — Total $ 1,303 $ 5 $ — $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of limited partnerships, private equity, and hedge funds. (4) Consists of property funds and trusts holding direct real estate investments. Fair Value Measurements Using Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 10 $ 10 $ — $ — Equity investments: Pooled funds (2) 281 — — — Fixed income investments: Derivatives 9 — 9 — Pooled funds (2) 782 — — — Other investments: Alternative investments (2) (3) 63 — — — Real estate (2) (4) 10 — — — Total $ 1,155 $ 10 $ 9 $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of limited partnerships, private equity, and hedge funds. (4) Consists of property funds and trusts holding direct real estate investments. Investment Policy and Strategy The U.S. investment policy, as established by the Aon Retirement Plan Governance and Investment Committee (“RPGIC”), seeks reasonable asset growth at prudent risk levels within weighted average target allocations. At December 31, 2019, the weighted average targeted allocation for the U.S. plans was 50% for equity investments, 29% for fixed income investments, and 21% for other investments. Aon believes that plan assets are well-diversified and are of appropriate quality. The investment portfolio asset allocation is reviewed quarterly and re-balanced to be within policy target allocations. The investment policy is reviewed at least annually and revised, as deemed appropriate by the RPGIC. The investment policies for international plans are generally established by the local pension plan trustees and seek to maintain the plans’ ability to meet liabilities and to comply with local minimum funding requirements. Plan assets are invested in diversified portfolios that provide adequate levels of return at an acceptable level of risk. The investment policies are reviewed at least annually and revised, as deemed appropriate to ensure that the objectives are being met. At December 31, 2019 , the weighted average targeted allocation for the U.K. and non-U.S. plans was 8% for equity investments, 82% for fixed income investments, and 10% for other investments. Cash Flows Contributions Based on current assumptions, in 2020 , the Company expects to contribute approximately $5 million , $99 million , and $19 million to its significant U.K., U.S., and other major pension plans, respectively. Estimated Future Benefit Payments Estimated future benefit payments for plans, not including voluntary one-time lump sum payments, are as follows at December 31, 2019 (in millions): U.K. U.S. Other 2020 $ 164 $ 186 $ 43 2021 $ 154 $ 190 $ 44 2022 $ 160 $ 192 $ 46 2023 $ 167 $ 189 $ 46 2024 $ 172 $ 181 $ 47 2025 – 2029 $ 913 $ 887 $ 252 U.S. and Canadian Other Postretirement Benefits The following table provides an overview of the accumulated projected benefit obligation, fair value of plan assets, funded status and net amount recognized as of December 31, 2019 and 2018 for the Company’s other significant postretirement benefit plans located in the U.S. and Canada (in millions): 2019 2018 Accumulated projected benefit obligation $ 103 $ 91 Fair value of plan assets 16 14 Funded status (87 ) (77 ) Unrecognized prior-service credit (1 ) (1 ) Unrecognized (gain) loss 3 (6 ) Net amount recognized $ (85 ) $ (84 ) Other information related to the Company’s other postretirement benefit plans are as follows: 2019 2018 2017 Net periodic benefit cost recognized (millions) $3 $3 $1 Weighted-average discount rate used to determine future benefit obligations 2.93 - 3.25% 3.91 - 4.26% 3.32 - 3.64% Weighted-average discount rate used to determine net periodic benefit costs 3.91 - 4.26% 3.32 - 3.64% 3.71 - 4.15% Based on current assumptions, the Company expects: • The amount in Accumulated other comprehensive income expected to be recognized as a component of net periodic benefit cost during 2020 is $0.2 million and $0.8 million of prior service credit and net gain, respectively. • To contribute $4 million to fund significant other postretirement benefit plans during 2020 . • Estimated future benefit payments will be approximately $5 million each year for 2020 through 2024, and $25 million in aggregate for 2025-2029. The accumulated postretirement benefit obligation is increased by $3 million and decreased by $2 million by a respective 1% increase or decrease to the assumed health care trend rate. The service cost and interest cost components of net periodic benefits cost is increased by $0.1 million and decreased by $0.1 million by a respective 1% increase or decrease to the assumed health care trend rate. For most of the participants in the U.S. plan, Aon’s liability for future plan cost increases for pre-65 and Medical Supplement plan coverage is limited to 5% per annum. Although the net employer trend rates range from 4% to 7% per year, because of this cap, these plans are effectively limited to 5% per year in the future.

Share-based Compensation Plans

Share-based Compensation Plans	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Share-based Compensation Plans	Share-based Compensation Plans The following table summarizes share-based compensation expense recognized in the Consolidated Statements of Income in Compensation and benefits (in millions): Years ended December 31 2019 2018 2017 Restricted share units (“RSUs”) $ 198 $ 186 $ 182 Performance share awards (“PSAs”) 110 143 127 Employee share purchase plans 9 9 10 Total share-based compensation expense 317 338 319 Tax benefit 66 74 73 Share-based compensation expense, net of tax $ 251 $ 264 $ 246 Restricted Share Units RSUs generally vest between three and five years . The fair value of RSUs is based upon the market value of Aon plc ordinary shares at the date of grant. With certain limited exceptions, any break in continuous employment will cause the forfeiture of all non-vested awards. Compensation expense associated with RSUs is recognized on a straight-line basis over the requisite service period. Dividend equivalents are paid on certain RSUs, based on the initial grant amount. The following table summarizes the status of the Company’s RSUs, including shares related to the Divested Business (shares in thousands, except fair value): 2019 2018 2017 Years ended December 31 Shares Fair Value at Date of Grant (1) Shares Fair Value at Date of Grant (1) Shares Fair Value at Date of Grant (1) Non-vested at beginning of year 4,208 $ 120 4,849 $ 104 6,195 $ 89 Granted 1,306 $ 175 1,500 $ 141 1,700 $ 123 Vested (1,661 ) $ 113 (1,943 ) $ 97 (2,407 ) $ 82 Forfeited (219 ) $ 131 (198 ) $ 114 (639 ) $ 93 Non-vested at end of year 3,634 $ 143 4,208 $ 120 4,849 $ 104 (1) Represents per share weighted average fair value of award at date of grant. The fair value of RSUs that vested during 2019 , 2018 and 2017 was $187 million , $189 million , and $197 million , respectively. Unamortized deferred compensation expense amounted to $349 million as of December 31, 2019 , with a remaining weighted average amortization period of approximately 2.1 years . Performance Share Awards The vesting of PSAs is contingent upon meeting a cumulative level of earnings per share related performance over a three -year period. The actual issuance of shares may range from 0 - 200% of the target number of PSAs granted, based on the terms of the plan and level of achievement of the related performance target. The grant date fair value of PSAs is based upon the market price of Aon plc ordinary shares at the date of grant. The performance conditions are not considered in the determination of the grant date fair value for these awards. Compensation expense is recognized over the performance period based on management’s estimate of the number of units expected to vest. Management evaluates its estimate of the actual number of shares expected to be issued at the end of the programs on a quarterly basis. The cumulative effect of the change in estimate is recognized in the period of change as an adjustment to Compensation and benefits in the Consolidated Statements of Income, if necessary. Dividend equivalents are not paid on PSAs. The following table summarizes the Company’s target PSAs granted and shares that would be issued at current performance levels for PSAs granted during the years ended December 31, 2019 , 2018 , and 2017 , respectively (shares in thousands and dollars in millions, except fair value): 2019 2018 2017 Target PSAs granted during period 467 564 548 Weighted average fair value per share at date of grant $ 165 $ 134 $ 114 Number of shares that would be issued based on current performance levels 453 818 1,057 Unamortized expense, based on current performance levels $ 50 $ 34 $ — During 2019 , the Company issued approximately 0.7 million shares in connection with performance achievements related to the 2016-2018 LPP. During 2018 , the Company issued approximately 1.0 million shares in connection with performance achievements related to the 2015-2017 LPP cycle. During 2017 , the Company issued approximately 0.9 million shares in connection with performance achievements related to the 2014-2016 LPP cycle.

Derivatives and Hedging

Derivatives and Hedging	12 Months Ended
	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives and Hedging	Derivatives and Hedging The Company is exposed to market risks, including changes in foreign currency exchange rates and interest rates. To manage the risk related to these exposures, the Company enters into various derivative instruments that reduce these risks by creating offsetting exposures. The Company does not enter into derivative transactions for trading or speculative purposes. Foreign Exchange Risk Management The Company is exposed to foreign exchange risk when it earns revenues, pays expenses, enters into monetary intercompany transfers or other transactions denominated in a currency that differs from its functional currency. The Company uses foreign exchange derivatives, typically forward contracts, options and cross-currency swaps, to reduce its overall exposure to the effects of currency fluctuations on cash flows. These exposures are hedged, on average, for less than two years . These derivatives are accounted for as hedges, and changes in fair value are recorded each period in Other comprehensive income (loss) in the Consolidated Statements of Comprehensive Income. The Company also uses foreign exchange derivatives, typically forward contracts and options, to economically hedge the currency exposure of the Company’s global liquidity profile, including monetary assets or liabilities that are denominated in a non-functional currency of an entity, typically on a rolling 30 -day basis, but may be for up to 1 year in the future. These derivatives are not accounted for as hedges, and changes in fair value are recorded each period in Other income (expense) in the Consolidated Statements of Income. The notional and fair values of derivative instruments are as follows (in millions): Notional Amount Net Amount of Derivative Assets Presented in the Statements of Financial Position (1) Net Amount of Derivative Liabilities Presented in the Statements of Financial Position (2) As of December 31 2019 2018 2019 2018 2019 2018 Foreign exchange contracts Accounted for as hedges $ 579 $ 646 $ 16 $ 17 $ 1 $ 2 Not accounted for as hedges (3) 297 269 2 1 — 6 Total $ 876 $ 915 $ 18 $ 18 $ 1 $ 8 (1) Included within Other current assets ( $7 million in 2019 and $3 million in 2018 ) or Other non-current assets ( $11 million in 2019 and $15 million in 2018 ). (2) Included within Other current liabilities ( $1 million in 2019 and $5 million in 2018 ) or Other non-current liabilities ( $3 million in 2018 ). (3) These contracts typically are for 30 -day durations and executed close to the last day of the most recent reporting month, thereby resulting in nominal fair values at the balance sheet date. The amounts of derivative gains (losses) recognized in the Consolidated Financial Statements are as follows (in millions): 2019 2018 2017 (Loss) Gain recognized in Accumulated other comprehensive loss $ (9 ) $ (18 ) $ 18 The amounts of derivative gains (losses) reclassified from Accumulated other comprehensive loss into Consolidated Statements of Income are as follows (in millions): Years Ended December 31 2019 2018 2017 Total revenue (1) $ (12 ) $ — $ — Compensation and benefits (1 ) 1 14 Other general expense — (2 ) (5 ) Interest expense (1 ) (2 ) (1 ) Other income (expense) (1) — (8 ) (9 ) Total $ (14 ) $ (11 ) $ (1 ) (1) With the adoption of new hedge accounting guidance in the first quarter of 2019, gains (losses) on derivatives accounted for as hedges are recognized in Total revenue in the Company’s Consolidated Statement of Income rather than Other income (expense). Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for additional details. The Company estimates that approximately $11 million of pretax losses currently included within Accumulated other comprehensive loss will be reclassified into earnings in the next twelve months. The Company recorded a loss of $18 million for 2019 , a loss of $27 million in 2018 , and a gain of $7 million in 2017 in Other income (expense) for foreign exchange derivatives not designated or qualifying as hedges. Net Investments in Foreign Operations Risk Management The Company uses non-derivative financial instruments to protect the value of its investments in a number of foreign subsidiaries. The Company has designated a portion of its euro-denominated commercial paper issuances as a non-derivative hedge of the foreign currency exposure of a net investment in its European operations. The change in fair value of the designated portion of the euro-denominated commercial paper due to changes in foreign currency exchange rates is recorded in Foreign currency translation adjustment, a component of Accumulated other comprehensive loss, to the extent it is effective as a hedge. The foreign currency translation adjustment of the hedged net investments is also recorded in Accumulated other comprehensive loss. Ineffective portions of net investment hedges, if any, are reclassified from Accumulated other comprehensive loss into earnings during the period of change. The Company had €101 million ( $112 million at December 31, 2019 exchange rates) and €220 million ( $250 million at December 31, 2018 exchange rates) of outstanding euro-denominated commercial paper at December 31, 2019 and 2018, respectively, designated as a hedge of the foreign currency exposure of its net investment in its European operations. The unrealized gain recognized in Accumulated other comprehensive loss related to the net investment non-derivative hedging instrument was $29 million and $21 million , as of December 31, 2019 and 2018, respectively. The Company did not reclassify any deferred gains or losses related to net investment hedges from Accumulated other comprehensive loss to earnings for 2019 , 2018 , and 2017 . In addition, the Company did not incur any ineffectiveness related to net investment hedges during 2019 , 2018 , and 2017 .

Fair Value Measurements and Fin

Fair Value Measurements and Financial Instruments	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements and Financial Instruments	Fair Value Measurements and Financial Instruments Accounting standards establish a three tier fair value hierarchy that prioritizes the inputs used in measuring fair values as follows: • Level 1 — observable inputs such as quoted prices for identical assets in active markets; • Level 2 — inputs other than quoted prices for identical assets in active markets, that are observable either directly or indirectly; and • Level 3 — unobservable inputs in which there is little or no market data which requires the use of valuation techniques and the development of assumptions. The following methods and assumptions are used to estimate the fair values of the Company’s financial instruments, including pension assets (refer to Note 13 “Employee Benefits”): Money market funds consist of institutional prime, treasury, and government money market funds. The Company reviews treasury and government money market funds to obtain reasonable assurance that the fund net asset value is $1 per share, and reviews the floating net asset value of institutional prime money market funds for reasonableness. Cash and cash equivalents consist of cash and institutional short-term investment funds. The Company reviews the short-term investment funds to obtain reasonable assurance that the fund net asset value is $1 per share. Equity investments consist of equity securities and equity derivatives valued using the closing stock price on a national securities exchange. Over the counter equity derivatives are valued using observable inputs such as underlying prices of the underlying security and volatility. On a sample basis the Company reviews the listing of Level 1 equity securities in the portfolio and agrees the closing stock prices to a national securities exchange, and independently verifies the observable inputs for Level 2 equity derivatives and securities. Fixed income investments consist of certain categories of bonds and derivatives. Corporate, government, and agency bonds are valued by pricing vendors who estimate fair value using recently executed transactions and proprietary models based on observable inputs, such as interest rate spreads, yield curves, and credit risk. Asset-backed securities are valued by pricing vendors who estimate fair value using discounted cash flow models utilizing observable inputs based on trade and quote activity of securities with similar features. Fixed income derivatives are valued by pricing vendors using observable inputs such as interest rates and yield curves. The Company obtains an understanding of the models, inputs, and assumptions used in developing prices provided by its vendors through discussions with the fund managers. The Company independently verifies the observable inputs, as well as assesses assumptions used for reasonableness based on relevant market conditions and internal Company guidelines. If an assumption is deemed unreasonable, based on the Company’s guidelines, it is then reviewed by management and the fair value estimate provided by the vendor is adjusted, if deemed appropriate. These adjustments do not occur frequently and historically are not material to the fair value estimates used in the Consolidated Financial Statements. Pooled funds consist of various equity, fixed income, commodity, and real estate mutual fund type investment vehicles. Pooled investment funds fair value is estimated based on the proportionate share ownership in the underlying net assets of the investment, which is based on the fair value of the underlying securities that trade on a national securities exchange. The Company gains an understanding of the investment guidelines and valuation policies of the fund and discusses fund performance with pooled fund managers. The Company obtains audited fund manager financial statements, when available. If the pooled fund is designed to replicate a publicly traded index, the Company compares the performance of the fund to the index to assess the reasonableness of the fair value measurement. Alternative investments consist of limited partnerships, private equity, and hedge funds. Alternative investment fair value is generally estimated based on the proportionate share ownership in the underlying net assets of the investment as determined by the general partner or investment manager. The valuations are based on various factors depending on investment strategy, proprietary models, and specific financial data or projections. The Company obtains audited fund manager financial statements, when available. The Company obtains a detailed understanding of the models, inputs, and assumptions used in developing prices provided by the investment managers, or appropriate party, through regular discussions. The Company also obtains the investment manger’s valuation policies and assesses the assumptions used for reasonableness based on relevant market conditions and internal Company guidelines. If an assumption is deemed unreasonable, based on the Company’s guidelines, it is then reviewed by management and the fair value estimate provided by the vendor is adjusted, if deemed appropriate. These adjustments do not occur frequently and historically are not material to the fair value estimates in the Consolidated Financial Statements. Derivatives are carried at fair value, based upon industry standard valuation techniques that use, where possible, current market-based or independently sourced pricing inputs, such as interest rates, currency exchange rates, or implied volatilities. Annuity contracts consist of insurance group annuity contracts purchased to match the pension benefit payment stream owed to certain selected plan participant demographics within a few major U.K. defined benefit plans. Annuity contracts are valued using a discounted cash flow model utilizing assumptions such as discount rate, mortality, and inflation. Real estate and REITs consist of publicly traded REITs and direct real estate investments. Level 1 REITs are valued using the closing stock price on a national securities exchange. Non-Level 1 values are based on the proportionate share of ownership in the underlying net asset value as determined by the investment manager. The Company independently reviews the listing of Level 1 REIT securities in the portfolio and agrees the closing stock prices to a national securities exchange. The Company gains an understanding of the investment guidelines and valuation policies of the non-Level 1 real estate funds and discusses performance with the fund managers. The Company obtains audited fund manager financial statements, when available. See the description of “Alternative investments” for further detail on valuation procedures surrounding non-Level 1 REITs. Debt is carried at outstanding principal balance, less any unamortized issuance costs, discount or premium. Fair value is based on quoted market prices or estimates using discounted cash flow analyses based on current borrowing rates for similar types of borrowing arrangements. The following tables present the categorization of the Company’s assets and liabilities that are measured at fair value on a recurring basis at December 31, 2019 and December 31, 2018 (in millions): Fair Value Measurements Using Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Money market funds (1) $ 2,007 $ 2,007 $ — $ — Other investments Government bonds $ 1 $ — $ 1 $ — Equity investments $ 1 $ — $ 1 $ — Derivatives (2) Gross foreign exchange contracts $ 21 $ — $ 21 $ — Liabilities Derivatives (2) Gross foreign exchange contracts $ 4 $ — $ 4 $ — Fair Value Measurements Using Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Money market funds (1) $ 1,759 $ 1,759 $ — $ — Other investments Government bonds $ 1 $ — $ 1 $ — Equity investments $ 2 $ — $ 2 $ — Derivatives (2) Gross foreign exchange contracts $ 21 $ — $ 21 $ — Liabilities Derivatives (2) Gross foreign exchange contracts $ 12 $ — $ 12 $ — (1) Included within Fiduciary assets or Short-term investments in the Consolidated Statements of Financial Position, depending on their nature and initial maturity. (2) Refer to Note 15 “Derivatives and Hedging” for additional information regarding the Company’s derivatives and hedging activity. There were no transfers of assets or liabilities between fair value hierarchy levels during 2019 or 2018 . The Company recognized no realized or unrealized gains or losses in the Consolidated Statements of Income related to assets and liabilities measured at fair value using unobservable inputs in 2019 , 2018 , or 2017 . The fair value of debt is classified as Level 2 of the fair value hierarchy. The following table provides the carrying value and fair value for the Company’s term debt (in millions): 2019 2018 As of December 31 Carrying Value Fair Value Carrying Value Fair Value Current portion of long-term debt $ 600 $ 614 $ — $ — Long-term debt $ 6,627 $ 7,442 $ 5,993 $ 6,159

Claims, Lawsuits and Other Cont

Claims, Lawsuits and Other Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Claims, Lawsuits, and Other Contingencies	Claims, Lawsuits, and Other Contingencies Legal Aon and its subsidiaries are subject to numerous claims, tax assessments, lawsuits and proceedings that arise in the ordinary course of business, which frequently include E&O claims. The damages claimed in these matters are or may be substantial, including, in many instances, claims for punitive, treble, or extraordinary damages. While Aon maintains meaningful E&O insurance and other insurance programs to provide protection against certain losses that arise in such matters, Aon has exhausted or materially depleted its coverage under some of the policies that protect the Company and, consequently, is self-insured or materially self-insured for some claims. Accruals for these exposures, and related insurance receivables, when applicable, are included in the Consolidated Statements of Financial Position and have been recognized in Other general expense in the Consolidated Statements of Income to the extent that losses are deemed probable and are reasonably estimable. These amounts are adjusted from time to time as developments warrant. Matters that are not probable and reasonably estimable are not accrued for in the Consolidated Financial Statements. The Company has included in the current matters described below certain matters in which (1) loss is probable, (2) loss is reasonably possible; that is, more than remote but not probable, or (3) there exists the reasonable possibility of loss greater than the accrued amount. In addition, the Company may from time to time disclose matters for which the probability of loss could be remote but the claim amounts associated with such matters are potentially significant. The reasonably possible range of loss for the matters described below for which loss is estimable, in excess of amounts that are deemed probable and estimable and therefore already accrued, is estimated to be between $0 and $0.1 billion , exclusive of any insurance coverage. These estimates are based on available information as of the date of this filing. As available information changes, the matters for which Aon is able to estimate may change, and the estimates themselves may change. In addition, many estimates involve significant judgment and uncertainty. For example, at the time of making an estimate, Aon may only have limited information about the facts underlying the claim and predictions and assumptions about future court rulings and outcomes may prove to be inaccurate. Although management at present believes that the ultimate outcome of all matters described below, individually or in the aggregate, will not have a material adverse effect on the consolidated financial position of Aon, legal proceedings are subject to inherent uncertainties and unfavorable rulings or other events. Unfavorable resolutions could include substantial monetary or punitive damages imposed on Aon or its subsidiaries. If unfavorable outcomes of these matters were to occur, future results of operations or cash flows for any particular quarterly or annual period could be materially adversely affected. Current Matters On October 3, 2017, Christchurch City Council (“CCC”) invoked arbitration to pursue a claim that it asserts against Aon New Zealand. Aon provided insurance broking services to CCC in relation to CCC’s 2010-2011 material damage and business interruption program. In December 2015, CCC settled its property and business interruption claim for its losses arising from the 2010-2011 Canterbury earthquakes against the underwriter of its material damage and business interruption program and the reinsurers of that underwriter. CCC contends that acts and omissions by Aon caused CCC to recover less in that settlement than it otherwise would have. CCC claims damages of approximately NZD 528 million ( $352 million at December 31, 2019 exchange rates) plus interest and costs. Aon believes that it has meritorious defenses and intends to vigorously defend itself against these claims. A retail insurance brokerage subsidiary of Aon was sued on September 6, 2018 in the United States District Court for the Southern District of New York by a client, Pilkington North America, Inc., that sustained damage from a tornado to its Ottawa, Illinois property. The lawsuit seeks between $45 million and $85 million in property and business interruption damages from either its insurer or Aon. The insurer contends that insurance proceeds were limited to $15 million in coverage by a windstorm sub-limit purportedly contained in the policy procured by Aon for Pilkington. The insurer therefore has tendered $15 million to Pilkington and denied coverage for the remainder of the loss. Pilkington sued the insurer and Aon seeking full coverage for the loss from the insurer or, in the alternative, seeking the same damages against Aon on various theories of professional liability if the court finds that the $15 million sub-limit applies to the claim. Aon believes it has meritorious defenses and intends to vigorously defend itself against these claims. In April 2017, the FCA announced an investigation relating to suspected competition law breaches in the aviation and aerospace broking industry, which, for Aon in 2016, represented less than $100 million in global revenue. The European Commission has now assumed jurisdiction over the investigation in place of the FCA. Other antitrust agencies outside the E.U. are also conducting formal or informal investigations regarding these matters. Aon intends to work diligently with all antitrust agencies concerned to ensure they can carry out their work as efficiently as possible. At this time, in light of the uncertainties and many variables involved, Aon cannot estimate the ultimate impact on our company from these investigations or any related private litigation, nor any damages, penalties, or fines related to them. There can be no assurance that the ultimate resolution of these matters will not have a material adverse effect on the Company’s consolidated financial position, results of operations, or liquidity. Settled/Closed Matters A pensions consulting and administration subsidiary of Aon provided advisory services to the Trustees of the Gleeds pension fund in the United Kingdom and, on occasion, to the relevant employer of the fund. In April 2014, the High Court, Chancery Division, London found that certain governing documents of the fund that sought to alter the fund’s benefit structure and that had been drafted by Aon were procedurally defective and therefore invalid. No lawsuit naming Aon as a party was filed, although a tolling agreement was entered. The High Court decision said that the additional liabilities in the pension fund resulting from the alleged defect in governing documents amounted to approximately £45 million ( $58 million at December 31, 2019 exchange rates). In December 2014, the Court of Appeal granted the employer leave to appeal the High Court decision. At a hearing in October 2016, the Court of Appeal approved a settlement of the pending litigation. On October 31, 2016, the fund’s trustees and employer sued Aon in the High Court, Chancery Division, London, alleging negligence and breach of duty in relation to the governing documents. The proceedings were served on Aon on December 20, 2016. The claimants sought damages of approximately £70 million ( $91 million at December 31, 2019 exchange rates) plus interest and costs. In February 2018, the claimants instructed new lawyers and added their previous lawyers as defendants to the Aon lawsuit. Claimants alleged that the previous lawyers were responsible for some of the losses sought from Aon because the lawyers gave negligent legal advice during the course of the High Court and Court of Appeal proceedings. In November 2019, the case was settled with no admission of liability by Aon. The terms of this settlement did not have a significant impact on Aon’s results of operations or financial condition. On June 29, 2015, Lyttelton Port Company Limited (“LPC”) sued Aon New Zealand in the Christchurch Registry of the High Court of New Zealand. LPC alleges, among other things, that Aon was negligent and in breach of contract in arranging LPC’s property insurance program for the period covering June 30, 2010, to June 30, 2011. LPC contended that acts and omissions by Aon caused LPC to recover less than it otherwise would have from insurers for losses suffered in the 2010 and 2011 Canterbury earthquakes. LPC claimed damages of approximately NZD $184 million ( $123 million at December 31, 2019 exchange rates) plus interest and costs. In April 2019, the case was settled with no admission of liability on the part of Aon. The terms of this settlement did not have a significant impact on Aon’s results of operations or financial condition. Guarantees and Indemnifications The Company provides a variety of guarantees and indemnifications to its customers and others. The maximum potential amount of future payments represents the notional amounts that could become payable under the guarantees and indemnifications if there were a total default by the guaranteed parties, without consideration of possible recoveries under recourse provisions or other methods. These amounts may bear no relationship to the expected future payments, if any, for these guarantees and indemnifications. Any anticipated amounts payable are included in the Consolidated Financial Statements and are recorded at fair value. The Company expects that, as prudent business interests dictate, additional guarantees and indemnifications may be issued from time to time. Redomestication In connection with the Redomestication, the Company on April 2, 2012 entered into various agreements pursuant to which it agreed to guarantee the obligations of its subsidiaries arising under issued and outstanding debt securities. Those agreements included the (1) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc, and The Bank of New York Mellon Trust Company, N.A., as trustee (the “Trustee”) (amending and restating the Indenture, dated as of September 10, 2010, between Aon Corporation and the Trustee), (2) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc and the Trustee (amending and restating the Indenture, dated as of December 16, 2002, between Aon Corporation and the Trustee), and (3) Amended and Restated Indenture, dated as of April 2, 2012, among Aon Corporation, Aon plc and the Trustee (amending and restating the Indenture, dated as of January 13, 1997, as supplemented by the First Supplemental Indenture, dated as of January 13, 1997). Sale of the Divested Business In connection with the sale of the Divested Business, the Company guaranteed future operating lease commitments related to certain facilities assumed by the Buyer. The Company is obligated to perform under the guarantees if the Divested Business defaults on the leases at any time during the remainder of the lease agreements, which expire on various dates through 2025. As of December 31, 2019 , the undiscounted maximum potential future payments under the lease guarantee were $70 million , with an estimated fair value of $12 million . No cash payments were made in connection to the lease commitments during the year ended December 31, 2019 . Additionally, the Company is subject to performance guarantee requirements under certain client arrangements that were assumed by the Buyer. Should the Divested Business fail to perform as required by the terms of the arrangements, the Company would be required to fulfill the remaining contract terms, which expire on various dates through 2023. As of December 31, 2019 , the undiscounted maximum potential future payments under the performance guarantees were $151 million , with an estimated fair value of $1 million . No cash payments were made in connection to the performance guarantees during the year ended December 31, 2019 . Letters of Credit Aon has entered into a number of arrangements whereby the Company’s performance on certain obligations is guaranteed by a third party through the issuance of LOCs. The Company had total LOCs outstanding of approximately $73 million at December 31, 2019 , compared to $83 million at December 31, 2018 . These LOCs cover the beneficiaries related to certain of Aon’s U.S. and Canadian non-qualified pension plan schemes and secure deductible retentions for Aon’s own workers compensation program. The Company has also obtained LOCs to cover contingent payments for taxes and other business obligations to third parties, and other guarantees for miscellaneous purposes at its international subsidiaries. Premium Payments The Company has certain contractual contingent guarantees for premium payments owed by clients to certain insurance companies. The maximum exposure with respect to such contractual contingent guarantees was approximately $110 million at December 31, 2019 , compared to $ 103 million at December 31, 2018 .

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2019
Segment Reporting [Abstract]
Segment Information	Segment Information The Company operates as one segment that includes all of Aon’s continuing operations, which as a global professional services firm provides advice and solutions to clients focused on risk, retirement, and health through five revenue lines which make up its principal products and services. The Chief Operating Decision Maker (the “CODM”) assesses the performance of the Company and allocates resources based on one segment: Aon United. The Company’s reportable operating segment has been determined using a management approach, which is consistent with the basis and manner in which Aon’s CODM uses financial information for the purposes of allocating resources and evaluating performance. The CODM assesses performance and allocates resources based on total Aon results against its key four metrics, including organic revenue growth, expense discipline, and collaborative behaviors that maximize value for Aon and its shareholders, regardless of which revenue line it benefits. As Aon operates as one segment, segment profit or loss is consistent with consolidated reporting as disclosed in the Consolidated Statements of Income. Refer to Note 3 “Revenue from Contracts with Customers” for further information on revenue by principal service line. Consolidated long-lived assets, net by geographic area are as follows (in millions): Years ended December 31 Total United States Americas other than U.S. United Kingdom Europe, Middle East, & Africa Asia Pacific 2019 (1) $ 1,550 $ 686 $ 145 $ 225 $ 319 $ 175 2018 $ 588 $ 288 $ 44 $ 58 $ 101 $ 97 (1) In the first quarter of 2019, Aon adopted new accounting guidance related to the treatment of leases which was applied under the modified retrospective approach. Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for further information.

Guarantee of Registered Securit

Guarantee of Registered Securities	12 Months Ended
	Dec. 31, 2019
Guarantee of Registered Securities
Guarantee of Registered Securities	Guarantee of Registered Securities As described in Note 17 “Claims, Lawsuits, and Other Contingencies,” in connection with the Redomestication, Aon plc entered into various agreements pursuant to which it agreed to guarantee the obligations of Aon Corporation arising under issued and outstanding debt securities, including the 5.00% Notes due September 2020, the 8.205% Notes due January 2027, and the 6.25% Notes due September 2040 (collectively, the “Aon Corp Notes”). Additionally, Aon plc has agreed to guarantee the obligations of Aon Corporation arising under the 4.50% Senior Notes due 2028, the 3.75% Senior Notes due 2029, and the 2.20% Senior Notes due 2022. Aon Corporation is a 100% indirectly owned subsidiary of Aon plc. All guarantees of Aon plc are full and unconditional. There are no other subsidiaries of Aon plc that are guarantors of the Aon Corp Notes. In addition, Aon Corporation entered into an agreement pursuant to which it agreed to guarantee the obligations of Aon plc arising under the 4.25% Notes due 2042 exchanged for Aon Corporation’s outstanding 8.205% Notes due January 2027 and has also agreed to guarantee the obligations of Aon plc arising under the 4.45% Notes due 2043, the 4.00% Notes due November 2023, the 2.875% Notes due May 2026, the 3.50% Notes due June 2024, the 4.60% Notes due June 2044, the 4.75% Notes due May 2045, the 2.80% Notes due March 2021, and the 3.875% Notes due December 2025 (collectively, the “Aon plc Notes”). In each case, the guarantee of Aon Corporation is full and unconditional. There are no subsidiaries of Aon plc, other than Aon Corporation, that are guarantors of the Aon plc Notes. As a result of the existence of these guarantees, the Company has elected to present the financial information set forth in this footnote in accordance with Rule 3-10 of Regulation S-X. In December 2018, Aon plc obtained direct ownership in two subsidiaries that were previously indirectly owned by Aon Corporation. In 2019, Aon Corporation obtained indirect ownership of subsidiaries that were previously indirectly owned by Aon plc. The financial results of both subsidiaries are included in the Other Non-Guarantor Subsidiaries column of the Condensed Consolidating Financial Statements. The Company has reflected this transfer on the Condensed Consolidating Statements of Income and Condensed Consolidating Statements of Comprehensive Income for the periods ended December 31, 2017 and December 31, 2018 and the Condensed Consolidating Statement of Financial Position for the period ended December 31, 2018. The following tables set forth Condensed Consolidating Statements of Income and Condensed Consolidating Statements of Comprehensive Income for the years ended December 31, 2019 , 2018 , and 2017 , Condensed Consolidating Statements of Financial Position as of December 31, 2019 , 2018 , and 2017 , and Condensed Consolidating Statements of Cash Flows for the years ended December 31, 2019 , 2018 , and 2017 , in accordance with Rule 3-10 of Regulation S-X. The condensed consolidating financial information includes the accounts of Aon plc, the accounts of Aon Corporation, and the combined accounts of the non-guarantor subsidiaries. The condensed consolidating financial statements are presented in all periods as a merger under common control. The principal consolidating adjustments are to eliminate the investment in subsidiaries and intercompany balances and transactions. Condensed Consolidating Statement of Income Year Ended December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Revenue Total revenue $ — $ — $ 11,013 $ — $ 11,013 Expenses Compensation and benefits 49 20 5,985 — 6,054 Information technology — — 494 — 494 Premises — 23 316 — 339 Depreciation of fixed assets — — 172 — 172 Amortization and impairment of intangible assets — — 392 — 392 Other general expense 10 4 1,379 — 1,393 Total operating expenses 59 47 8,738 — 8,844 Operating income (loss) (59 ) (47 ) 2,275 — 2,169 Interest income — 39 — (31 ) 8 Interest expense (188 ) (133 ) (17 ) 31 (307 ) Intercompany interest income (expense) 21 (464 ) 443 — — Intercompany other income (expense) 185 (362 ) 177 — — Other income (expense) 18 (60 ) 51 (8 ) 1 Income (loss) from continuing operations before income taxes (23 ) (1,027 ) 2,929 (8 ) 1,871 Income tax expense (benefit) (12 ) (182 ) 491 — 297 Net income (loss) from continuing operations (11 ) (845 ) 2,438 (8 ) 1,574 Net income (loss) from discontinued operations — — (1 ) — (1 ) Net income (loss) before equity in earnings of subsidiaries (11 ) (845 ) 2,437 (8 ) 1,573 Equity in earnings of subsidiaries 1,551 1,254 409 (3,214 ) — Net income 1,540 409 2,846 (3,222 ) 1,573 Less: Net income attributable to noncontrolling interests — — 41 — 41 Net income attributable to Aon shareholders $ 1,540 $ 409 $ 2,805 $ (3,222 ) $ 1,532 Condensed Consolidating Statement of Income Year Ended December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Revenue Total revenue $ — $ — $ 10,770 $ — $ 10,770 Expenses Compensation and benefits 74 (1 ) 6,030 — 6,103 Information technology — — 484 — 484 Premises — — 370 — 370 Depreciation of fixed assets — — 176 — 176 Amortization and impairment of intangible assets — — 593 — 593 Other general expense 4 90 1,406 — 1,500 Total operating expenses 78 89 9,059 — 9,226 Operating income (loss) (78 ) (89 ) 1,711 — 1,544 Interest income — 58 — (53 ) 5 Interest expense (203 ) (101 ) (27 ) 53 (278 ) Intercompany interest income (expense) 15 (514 ) 499 — — Intercompany other income (expense) 65 (373 ) 308 — — Other income (expense) 41 (48 ) 3 (21 ) (25 ) Income (loss) from continuing operations before income taxes (160 ) (1,067 ) 2,494 (21 ) 1,246 Income tax expense (benefit) (60 ) (192 ) 398 — 146 Net income (loss) from continuing operations (100 ) (875 ) 2,096 (21 ) 1,100 Net income (loss) from discontinued operations — — 74 — 74 Net income (loss) before equity in earnings of subsidiaries (100 ) (875 ) 2,170 (21 ) 1,174 Equity in earnings of subsidiaries 1,255 1,009 134 (2,398 ) — Net income 1,155 134 2,304 (2,419 ) 1,174 Less: Net income attributable to noncontrolling interests — — 40 — 40 Net income attributable to Aon shareholders $ 1,155 $ 134 $ 2,264 $ (2,419 ) $ 1,134 Condensed Consolidating Statement of Income Year Ended December 31, 2017 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Revenue Total revenue $ — $ — $ 9,998 $ — $ 9,998 Expenses Compensation and benefits 150 26 5,827 — 6,003 Information technology — — 419 — 419 Premises — — 348 — 348 Depreciation of fixed assets — — 187 — 187 Amortization and impairment of intangible assets — — 704 — 704 Other general expense 12 20 1,240 — 1,272 Total operating expenses 162 46 8,725 — 8,933 Operating income (loss) (162 ) (46 ) 1,273 — 1,065 Interest income — 52 4 (29 ) 27 Interest expense (202 ) (94 ) (15 ) 29 (282 ) Intercompany interest income (expense) 14 (543 ) 529 — — Intercompany other income (expense) 282 (385 ) 103 — — Other income (expense) (62 ) 12 (93 ) 18 (125 ) Income (loss) from continuing operations before income taxes (130 ) (1,004 ) 1,801 18 685 Income tax expense (benefit) (43 ) (110 ) 403 — 250 Net income (loss) from continuing operations (87 ) (894 ) 1,398 18 435 Net income (loss) from discontinued operations — — 828 — 828 Net income (loss) before equity in earnings of subsidiaries (87 ) (894 ) 2,226 18 1,263 Equity in earnings of subsidiaries 1,295 1,141 247 (2,683 ) — Net income 1,208 247 2,473 (2,665 ) 1,263 Less: Net income attributable to noncontrolling interests — — 37 — 37 Net income attributable to Aon shareholders $ 1,208 $ 247 $ 2,436 $ (2,665 ) $ 1,226 Condensed Consolidating Statement of Comprehensive Income Year Ended December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Net income $ 1,540 $ 409 $ 2,846 $ (3,222 ) $ 1,573 Less: Net income attributable to noncontrolling interests — — 41 — 41 Net income attributable to Aon shareholders 1,540 409 2,805 (3,222 ) 1,532 Other comprehensive income (loss), net of tax: Change in fair value of financial instruments — 3 — — 3 Foreign currency translation adjustments — — 6 8 14 Postretirement benefit obligation — (44 ) (97 ) — (141 ) Total other comprehensive income (loss) — (41 ) (91 ) 8 (124 ) Equity in other comprehensive income (loss) of subsidiaries, net of tax (132 ) (131 ) (172 ) 435 — Less: Other comprehensive income (loss) attributable to noncontrolling interests — — — — — Total other comprehensive income (loss) attributable to Aon shareholders (132 ) (172 ) (263 ) 443 (124 ) Comprehensive income (loss) attributable to Aon shareholders $ 1,408 $ 237 $ 2,542 $ (2,779 ) $ 1,408 Condensed Consolidating Statement of Comprehensive Income Year Ended December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Net income $ 1,155 $ 134 $ 2,304 $ (2,419 ) $ 1,174 Less: Net income attributable to noncontrolling interests — — 40 — 40 Net income attributable to Aon shareholders 1,155 134 2,264 (2,419 ) 1,134 Other comprehensive income (loss), net of tax: Change in fair value of financial instruments — — 11 — 11 Foreign currency translation adjustments — — (465 ) 21 (444 ) Postretirement benefit obligation — (2 ) 19 — 17 Total other comprehensive income (loss) — (2 ) (435 ) 21 (416 ) Equity in other comprehensive income (loss) of subsidiaries, net of tax (433 ) (415 ) (417 ) 1,265 — Less: Other comprehensive income (loss) attributable to noncontrolling interests — — (4 ) — (4 ) Total other comprehensive income (loss) attributable to Aon shareholders (433 ) (417 ) (848 ) 1,286 (412 ) Comprehensive income (loss) attributable to Aon shareholders $ 722 $ (283 ) $ 1,416 $ (1,133 ) $ 722 Condensed Consolidating Statement of Comprehensive Income Year Ended December 31, 2017 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Net income $ 1,208 $ 247 $ 2,473 $ (2,665 ) $ 1,263 Less: Net income attributable to noncontrolling interests — — 37 — 37 Net income attributable to Aon shareholders 1,208 247 2,436 (2,665 ) 1,226 Other comprehensive income (loss), net of tax: Change in fair value of financial instruments — 3 9 — 12 Foreign currency translation adjustments — — 408 (18 ) 390 Postretirement benefit obligation — (101 ) 120 — 19 Total other comprehensive income (loss) — (98 ) 537 (18 ) 421 Equity in other comprehensive income (loss) of subsidiaries, net of tax 434 515 417 (1,366 ) — Less: Other comprehensive income (loss) attributable to noncontrolling interests — — 5 — 5 Total other comprehensive income (loss) attributable to Aon shareholders 434 417 949 (1,384 ) 416 Comprehensive income (loss) attributable to Aon shareholders $ 1,642 $ 664 $ 3,385 $ (4,049 ) $ 1,642 Condensed Consolidating Statement of Financial Position As of December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Assets Current assets Cash and cash equivalents $ — $ 2,271 $ 468 $ (1,949 ) $ 790 Short-term investments — 28 110 — 138 Receivables, net — — 3,112 — 3,112 Fiduciary assets — — 11,834 — 11,834 Intercompany receivables 246 1,214 12,799 (14,259 ) — Other current assets — 7 595 — 602 Total current assets 246 3,520 28,918 (16,208 ) 16,476 Goodwill — — 8,165 — 8,165 Intangible assets, net — — 783 — 783 Operating lease right-of-use assets — 110 819 — 929 Fixed assets, net — — 621 — 621 Deferred tax assets 89 577 169 (190 ) 645 Intercompany receivables 868 261 7,046 (8,175 ) — Prepaid pension — 7 1,209 — 1,216 Other non-current assets — 32 538 — 570 Investment in subsidiary 8,899 19,470 (957 ) (27,412 ) — Total assets $ 10,102 $ 23,977 $ 47,311 $ (51,985 ) $ 29,405 Liabilities and equity Liabilities Current liabilities Accounts payable and accrued liabilities $ 2,157 $ 56 $ 1,675 $ (1,949 ) $ 1,939 Short-term debt and current portion of long-term debt 112 600 — — 712 Fiduciary liabilities — — 11,834 — 11,834 Intercompany payables 234 12,978 1,047 (14,259 ) — Other current liabilities — 80 1,006 — 1,086 Total current liabilities 2,503 13,714 15,562 (16,208 ) 15,571 Long-term debt 4,223 2,404 — — 6,627 Non-current operating lease liabilities — 143 801 — 944 Deferred tax liabilities — — 389 (190 ) 199 Pension, other postretirement, and other post-employment liabilities — 1,348 390 — 1,738 Intercompany payables — 7,212 963 (8,175 ) — Other non-current liabilities 1 113 763 — 877 Total liabilities 6,727 24,934 18,868 (24,573 ) 25,956 Equity Total Aon shareholders' equity 3,375 (957 ) 28,369 (27,412 ) 3,375 Noncontrolling interests — — 74 — 74 Total equity 3,375 (957 ) 28,443 (27,412 ) 3,449 Total liabilities and equity $ 10,102 $ 23,977 $ 47,311 $ (51,985 ) $ 29,405 Condensed Consolidating Statement of Financial Position As of December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Assets Current assets Cash and cash equivalents $ — $ 862 $ 575 $ (781 ) $ 656 Short-term investments — 56 116 — 172 Receivables, net — — 2,760 — 2,760 Fiduciary assets — — 10,166 — 10,166 Intercompany receivables 191 897 11,634 (12,722 ) — Other current assets — 16 602 — 618 Total current assets 191 1,831 25,853 (13,503 ) 14,372 Goodwill — — 8,171 — 8,171 Intangible assets, net — — 1,149 — 1,149 Fixed assets, net — — 588 — 588 Deferred tax assets 94 467 144 (144 ) 561 Intercompany receivables 403 261 7,225 (7,889 ) — Prepaid pension — 5 1,128 — 1,133 Other non-current assets 1 30 417 — 448 Investment in subsidiary 8,433 19,031 (983 ) (26,481 ) — Total assets $ 9,122 $ 21,625 $ 43,692 $ (48,017 ) $ 26,422 Liabilities and equity Liabilities Current liabilities Accounts payable and accrued liabilities $ 274 $ 70 $ 2,380 $ (781 ) $ 1,943 Short-term debt and current portion of long-term debt 250 — 1 — 251 Fiduciary liabilities — — 10,166 — 10,166 Intercompany payables 213 11,875 634 (12,722 ) — Other current liabilities — 69 867 — 936 Total current liabilities 737 12,014 14,048 (13,503 ) 13,296 Long-term debt 4,231 1,762 — — 5,993 Deferred tax liabilities — — 325 (144 ) 181 Pension, other postretirement, and other post-employment liabilities — 1,275 361 — 1,636 Intercompany payables — 7,390 499 (7,889 ) — Other non-current liabilities 3 167 927 — 1,097 Total liabilities 4,971 22,608 16,160 (21,536 ) 22,203 Equity Total Aon shareholders' equity 4,151 (983 ) 27,464 (26,481 ) 4,151 Noncontrolling interests — — 68 — 68 Total equity 4,151 (983 ) 27,532 (26,481 ) 4,219 Total liabilities and equity $ 9,122 $ 21,625 $ 43,692 $ (48,017 ) $ 26,422 Condensed Consolidating Statement of Cash Flows Year Ended December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Cash flows from operating activities Cash provided by (used for) operating activities - continuing operations $ 280 $ (140 ) $ 2,408 $ (713 ) $ 1,835 Cash provided by operating activities - discontinued operations — — — — — Cash provided by (used for) operating activities 280 (140 ) 2,408 (713 ) 1,835 Cash flows from investing activities Proceeds from investments — 29 189 (157 ) 61 Payments for investments — (62 ) (52 ) 1 (113 ) Net sales (purchases) of short-term investments — non-fiduciary — 28 7 — 35 Acquisition of businesses, net of cash acquired — — (39 ) — (39 ) Sale of businesses, net of cash sold — — 52 — 52 Capital expenditures — — (225 ) — (225 ) Cash used for investing activities - continuing operations — (5 ) (68 ) (156 ) (229 ) Cash used for investing activities - discontinued operations — — — — — Cash used for investing activities — (5 ) (68 ) (156 ) (229 ) Cash flows from financing activities Share repurchase (1,960 ) — — — (1,960 ) Advances from (to) affiliates 2,350 314 (2,365 ) (299 ) — Issuance of shares for employee benefit plans (131 ) — — — (131 ) Issuance of debt 2,739 3,313 — — 6,052 Repayment of debt (2,868 ) (2,073 ) — — (4,941 ) Cash dividends to shareholders (410 ) — — — (410 ) Noncontrolling interests and other financing activities — — (103 ) — (103 ) Cash provided by (used for) financing activities - continuing operations (280 ) 1,554 (2,468 ) (299 ) (1,493 ) Cash used for financing activities - discontinued operations — — — — — Cash provided by (used for) financing activities (280 ) 1,554 (2,468 ) (299 ) (1,493 ) Effect of exchange rate changes on cash and cash equivalents — — 21 — 21 Net increase (decrease) in cash and cash equivalents — 1,409 (107 ) (1,168 ) 134 Cash and cash equivalents at beginning of year — 862 575 (781 ) 656 Cash and cash equivalents at end of year $ — $ 2,271 $ 468 $ (1,949 ) $ 790 Condensed Consolidating Statement of Cash Flows Year Ended December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Cash flows from operating activities Cash provided by operating activities - continuing operations $ 1,575 $ 3 $ 3,608 $ (3,500 ) $ 1,686 Cash provided by operating activities - discontinued operations — — — — — Cash provided by operating activities 1,575 3 3,608 (3,500 ) 1,686 Cash flows from investing activities Proceeds from investments — 24 955 (908 ) 71 Payments for investments (13 ) (47 ) (33 ) 13 (80 ) Net sales (purchases) of short-term investments — non-fiduciary — 299 49 — 348 Acquisition of businesses, net of cash acquired — — (58 ) — (58 ) Sale of businesses, net of cash sold — — (10 ) — (10 ) Capital expenditures — — (240 ) — (240 ) Cash provided by (used for) investing activities - continuing operations (13 ) 276 663 (895 ) 31 Cash used for investing activities - discontinued operations — — — — — Cash provided by (used for) investing activities (13 ) 276 663 (895 ) 31 Cash flows from financing activities Share repurchase (1,470 ) — — — (1,470 ) Advances from (to) affiliates 156 (2,291 ) (4,041 ) 6,176 — Issuance of shares for employee benefit plans (149 ) — — — (149 ) Issuance of debt 1,723 4,028 3 — 5,754 Repayment of debt (1,441 ) (3,678 ) (298 ) — (5,417 ) Cash dividends to shareholders (382 ) — — — (382 ) Noncontrolling interests and other financing activities — — (35 ) — (35 ) Cash used for financing activities - continuing operations (1,563 ) (1,941 ) (4,371 ) 6,176 (1,699 ) Cash used for financing activities - discontinued operations — — — — — Cash used for financing activities (1,563 ) (1,941 ) (4,371 ) 6,176 (1,699 ) Effect of exchange rate changes on cash and cash equivalents — — (118 ) — (118 ) Net decrease in cash and cash equivalents (1 ) (1,662 ) (218 ) 1,781 (100 ) Cash and cash equivalents at beginning of year 1 2,524 793 (2,562 ) 756 Cash and cash equivalents at end of year $ — $ 862 $ 575 $ (781 ) $ 656 Condensed Consolidating Statement of Cash Flows Year Ended December 31, 2017 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Cash flows from operating activities Cash provided by operating activities - continuing operations $ 2,787 $ 503 $ 2,010 $ (4,631 ) $ 669 Cash provided by operating activities - discontinued operations — — 65 — 65 Cash provided by operating activities 2,787 503 2,075 (4,631 ) 734 Cash flows from investing activities Proceeds from investments 224 587 582 (1,325 ) 68 Payments for investments (261 ) (29 ) (576 ) 802 (64 ) Net sales (purchases) of short-term investments — non-fiduciary — (215 ) (17 ) — (232 ) Acquisition of businesses, net of cash acquired — — (1,029 ) — (1,029 ) Sale of businesses, net of cash sold — — 4,246 — 4,246 Capital expenditures — — (183 ) — (183 ) Cash provided by (used for) investing activities - continuing operations (37 ) 343 3,023 (523 ) 2,806 Cash used for investing activities - discontinued operations — — (19 ) — (19 ) Cash provided by (used for) investing activities (37 ) 343 3,004 (523 ) 2,787 Cash flows from financing activities Share repurchase (2,399 ) — — — (2,399 ) Advances from (to) affiliates 426 95 (4,975 ) 4,454 — Issuance of shares for employee benefit plans (121 ) — — — (121 ) Issuance of debt 544 1,100 10 — 1,654 Repayment of debt (835 ) (1,150 ) (14 ) — (1,999 ) Cash dividends to shareholders (364 ) — — — (364 ) Noncontrolling interests and other financing activities — — (36 ) — (36 ) Cash provided by (used for) financing activities - continuing operations (2,749 ) 45 (5,015 ) 4,454 (3,265 ) Cash used for financing activities - discontinued operations — — — — — Cash provided by (used for) financing activities (2,749 ) 45 (5,015 ) 4,454 (3,265 ) Effect of exchange rate changes on cash and cash equivalents — — 69 — 69 Net increase in cash and cash equivalents 1 891 133 (700 ) 325 Cash and cash equivalents at beginning of year (1) — 1,633 660 (1,862 ) 431 Cash and cash equivalents at end of year $ 1 $ 2,524 $ 793 $ (2,562 ) $ 756 (1) Includes $5 million of discontinued operations at December 31, 2016

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) Selected quarterly financial data for the years ended December 31, 2019 and 2018 are as follows (in millions, except per share data): 1Q 2Q 3Q 4Q 2019 Income Statement Data Total revenue $ 3,143 $ 2,606 $ 2,379 $ 2,885 $ 11,013 Operating income 872 413 360 524 2,169 Net income from continuing operations 676 287 229 382 1,574 Income from discontinued operations, net of tax — — (1 ) — (1 ) Net income 676 287 228 382 1,573 Less: Net income attributable to noncontrolling interests 17 10 6 8 41 Net income attributable to Aon shareholders $ 659 $ 277 $ 222 $ 374 $ 1,532 Per Share Data Basic net income per share attributable to Aon shareholders Continuing operations $ 2.72 $ 1.15 $ 0.94 $ 1.59 $ 6.42 Discontinued operations — — — — — Net income $ 2.72 $ 1.15 $ 0.94 $ 1.59 $ 6.42 Diluted net income per share attributable to Aon shareholders Continuing operations $ 2.70 $ 1.14 $ 0.93 $ 1.58 $ 6.37 Discontinued operations — — — — — Net income $ 2.70 $ 1.14 $ 0.93 $ 1.58 $ 6.37 1Q 2Q 3Q 4Q 2018 Income Statement Data Total revenue $ 3,090 $ 2,561 $ 2,349 $ 2,770 $ 10,770 Operating income 799 (16 ) 262 499 1,544 Net income from continuing operations 604 57 155 284 1,100 Income from discontinued operations, net of tax 6 1 (2 ) 69 74 Net income 610 58 153 353 1,174 Less: Net income attributable to noncontrolling interests 16 10 6 8 40 Net income attributable to Aon shareholders $ 594 $ 48 $ 147 $ 345 $ 1,134 Per Share Data Basic net income per share attributable to Aon shareholders Continuing operations $ 2.37 $ 0.19 $ 0.61 $ 1.14 $ 4.32 Discontinued operations 0.02 0.01 (0.01 ) 0.28 0.30 Net income $ 2.39 $ 0.20 $ 0.60 $ 1.42 $ 4.62 Diluted net income per share attributable to Aon shareholders Continuing operations $ 2.35 $ 0.19 $ 0.61 $ 1.13 $ 4.29 Discontinued operations 0.02 — (0.01 ) 0.28 0.30 Net income $ 2.37 $ 0.19 $ 0.60 $ 1.41 $ 4.59

Summary of Significant Accoun_2

Summary of Significant Accounting Principles and Practices (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Accounting	The accompanying Consolidated Financial Statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”). The Consolidated Financial Statements include the accounts of Aon plc and all of its controlled subsidiaries (“Aon” or the “Company”). Intercompany accounts and transactions have been eliminated. The Consolidated Financial Statements include, in the opinion of management, all adjustments (consisting of normal recurring adjustments and reclassifications) necessary to present fairly the Company’s consolidated financial position, results of operations and cash flows for all periods presented.
Use of Estimates	Use of Estimates The preparation of the accompanying Consolidated Financial Statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosures of contingent assets and liabilities at the date of the Consolidated Financial Statements, and the reported amounts of reserves and expenses. These estimates and assumptions are based on management’s best estimates and judgments. Management evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors, including the current economic environment. Management believes its estimates to be reasonable given the current facts available. Aon adjusts such estimates and assumptions when facts and circumstances dictate. Illiquid credit markets, volatile equity markets, and foreign currency exchange rate movements increase the uncertainty inherent in such estimates and assumptions. As future events and their effects cannot be determined with precision, actual results could differ significantly from these estimates. Changes in estimates resulting from continuing changes in the economic environment would, if applicable, be reflected in the Consolidated Financial Statements in future periods.
Revenue Recognition	Revenue Recognition The Company generates revenues primarily through commissions, compensation from insurance and reinsurance companies for services provided to them, and fees from customers. Commissions and fees for brokerage services vary depending upon several factors, which may include the amount of premium, the type of insurance or reinsurance coverage provided, the particular services provided to a client, insurer, or reinsurer, and the capacity in which the Company acts. Compensation from insurance and reinsurance companies includes: (1) fees for consulting and analytics services and (2) fees and commissions for administrative and other services provided to or on behalf of insurers. In Aon’s capacity as an insurance and reinsurance broker, the service promised to the customer is placement of an effective insurance or reinsurance policy, respectively. At the completion of the insurance or reinsurance policy placement process once coverage is effective, the customer has obtained control over the services promised by the Company. Judgment is not typically required when assessing whether the coverage is effective. Fees from clients for advice and consulting services are dependent on the extent and value of the services provided. Payment terms for the Company’s principal service lines are discussed below; the Company believes these terms are consistent with current industry practices. Significant financing components are typically not present in Aon’s arrangements. The Company recognizes revenue when control of the promised services is transferred to the customer in the amount that best reflects the consideration to which the Company expects to be entitled in exchange for those services. For arrangements where control is transferred over time, an input or output method is applied that represents a faithful depiction of the progress towards completion of the performance obligation. For arrangements that include variable consideration, the Company assesses whether any amounts should be constrained. For arrangements that include multiple performance obligations, the Company allocates consideration based on their relative fair values. Costs incurred by the Company in obtaining a contract are capitalized and amortized on a systematic basis that is consistent with the transfer of control of the services to which the asset relates, considering anticipated renewals when applicable. Certain contract related costs, including pre-placement brokerage costs, are capitalized as a cost to fulfill and are amortized on a systematic basis consistent with the transfer of control of the services to which the asset relates, which is generally less than one year. The Company has elected to apply practical expedients to not disclose the revenue related to unsatisfied performance obligations if (1) the contract has an original duration of 1 year or less, (2) the Company has recognized revenue for the amount in which it has the right to bill, and (3) the variable consideration is allocated entirely to an unsatisfied performance obligation which is recognized as a series of distinct goods or services that form a single performance obligation. Disaggregation of Revenue The following is a description of principal service lines from which the Company generates its revenue: Commercial Risk Solutions includes retail brokerage, cyber solutions, global risk consulting, and captives. Revenue primarily includes insurance commissions and fees for services rendered. Revenue is predominantly recognized at a point in time upon the effective date of the underlying policy (or policies), or for a limited number of arrangements, over the term of the arrangement using output measures to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services. For arrangements recognized over time, various output measures, including units transferred and time elapsed, are utilized to provide a faithful depiction of the progress towards completion of the performance obligation. Revenue is recorded net of allowances for estimated policy cancellations, which are determined based on an evaluation of historical and current cancellation data. Commissions and fees for brokerage services may be invoiced near the effective date of the underlying policy or over the term of the arrangement in installments during the policy period. Reinsurance Solutions includes treaty and facultative reinsurance brokerage and capital markets. Revenue primarily includes reinsurance commissions and fees for services rendered. Revenue is predominantly recognized at a point in time upon the effective date of the underlying policy (or policies), or for a limited number of arrangements, over the term of the arrangement using output measures to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services. For arrangements recognized over time, various output measures, including units delivered and time elapsed, are utilized to provide a faithful depiction of the progress towards completion of the performance obligation. Commissions and fees for brokerage services may be invoiced at the inception of the reinsurance period for certain reinsurance brokerage, or more commonly, over the term of the arrangement in installments based on deposit or minimum premiums for most treaty reinsurance arrangements. Retirement Solutions includes core retirement, investment consulting, and human capital. Revenue consists primarily of fees paid by customers for consulting services, such as risk management strategies, health and benefits, and human capital consulting services. Revenue recognized for these arrangements is predominantly recognized over the term of the arrangement using input or output measures to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services, or for certain arrangements, at a point in time upon completion of the services. For consulting arrangements recognized over time, revenue will be recognized based on a measure of progress that depicts the transfer of control of the services to the customer, utilizing an appropriate input or output measure to provide a reasonable assessment of the progress towards completion of the performance obligation including units delivered or time elapsed. Fees paid by customers for consulting services are typically charged on an hourly, project or fixed-fee basis, and revenue for these arrangements is typically recognized based on time incurred, days elapsed, or reports delivered. Revenue from time-and-materials or cost-plus arrangements are recognized as services are performed using input or output measures to provide a reasonable assessment of the progress towards completion of the performance obligation including hours worked, and revenue for these arrangements is typically recognized based on time and materials incurred. Reimbursements received for out-of-pocket expenses are recorded as a component of revenue. Payment terms vary but are typically over the contract term in installments. Health Solutions includes health and benefits brokerage and health care exchanges. Revenue primarily includes insurance commissions and fees for services rendered. For brokerage commissions, revenue is predominantly recognized at the effective date of the underlying policy (or policies), or for a limited number of arrangements, over the term of the arrangement to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services using input or output measures, including units delivered or time elapsed, to provide a faithful depiction of the progress towards completion of the performance obligation. Revenue from health care exchange arrangements are typically recognized upon successful enrollment of participants, net of a reserve for estimated cancellations. Commissions and fees for brokerage services may be invoiced at the effective date of the underlying policy or over the term of the arrangement in installments during the policy period. Payment terms for other services vary but are typically over the contract term in installments. Data & Analytic Services includes Affinity, Aon InPoint, and ReView. Revenue consists primarily of fees for services rendered and is generally recognized over the term of the arrangement to depict the transfer of control of the services to customers in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those services. Payment terms vary but are typically over the contract term in installments. For arrangements recognized over time, revenue will be recognized based on a measure of progress that depicts the transfer of control of the services to the customer, utilizing an appropriate input or output measure to provide a faithful depiction of the progress towards completion of the performance obligation, including units delivered or time elapsed. Input and output measures utilized vary based on the arrangement but typically include reports provided or days elapsed.
Share-based Compensation Expense	Share-based Compensation Expense Share-based payments to employees, including grants of restricted share units and performance share awards, are measured based on grant date fair value. The Company recognizes compensation expense over the requisite service period for awards expected to ultimately vest. Forfeitures are estimated on the date of grant and revised if actual or expected forfeiture activity differs materially from original estimates.
Pension and Other Post-Retirement Benefits	Pension and Other Postretirement Benefits The Company records net periodic cost relating to its pension and other postretirement benefit plans based on calculations that include various actuarial assumptions, including discount rates, assumed rates of return on plan assets, inflation rates, mortality rates, compensation increases, and turnover rates. The Company reviews its actuarial assumptions on an annual basis and modifies these assumptions based on current rates and trends. The effects of gains, losses, and prior service costs and credits are amortized over future service periods or future estimated lives if the plans are frozen as reflected in Other income (expense) within the Consolidated Statements of Income. The funded status of each plan, calculated as the fair value of plan assets less the benefit obligation, is reflected in the Company’s Consolidated Statements of Financial Position using a December 31 measurement date.
Net Income per Share	Net Income per Share Basic net income per share is computed by dividing net income available to ordinary shareholders by the weighted-average number of ordinary shares outstanding, including participating securities, which consist of unvested share awards with non-forfeitable rights to dividends. Diluted net income per share is computed by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding, which have been adjusted for the dilutive effect of potentially issuable ordinary shares, including certain contingently issuable shares. The diluted earnings per share calculation reflects the more dilutive effect of either (1) the two-class method that assumes that the participating securities have not been exercised, or (2) the treasury stock method. Potentially issuable shares are not included in the computation of diluted income per share if their inclusion would be antidilutive.
Cash and Cash Equivalents and Short-term Investments	Cash and Cash Equivalents and Short-term Investments Cash and cash equivalents include cash balances and all highly liquid investments with initial maturities of three months or less. Short-term investments consist of money market funds. The estimated fair value of Cash and cash equivalents and Short-term investments approximates their carrying values.
Fiduciary Assets and Liabilities	Fiduciary Assets and Liabilities In its capacity as an insurance agent and broker, Aon collects premiums from insureds and, after deducting its commission, remits the premiums to the respective insurers. Aon also collects claims or refunds from insurers on behalf of insureds. Uncollected premiums from insureds and uncollected claims or refunds from insurers are recorded as Fiduciary assets in the Company’s Consolidated Statements of Financial Position. Unremitted insurance premiums and claims are held in a fiduciary capacity and the obligation to remit these funds is recorded as Fiduciary liabilities in the Consolidated Statements of Financial Position.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts
Fixed Assets	.
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill represents the excess of acquisition cost over the fair value of the net assets acquired in the acquisition of a business. Goodwill is allocated to applicable reporting units. Upon disposition of a business entity, goodwill is allocated to the disposed entity based on the fair value of that entity compared to the fair value of the reporting unit in which it was included. Goodwill is not amortized, but instead is tested for impairment at least annually. The goodwill impairment test is performed at the reporting unit level. The Company may initially perform a qualitative analysis to determine if it is more likely than not that the goodwill balance is impaired. If a qualitative assessment is not performed or if a determination is made that it is not more likely than not that their value of the reporting unit exceeds its carrying amount, then the Company will perform a two-step quantitative analysis. First, the fair value of each reporting unit is compared to its carrying value. If the fair value of the reporting unit is less than its carrying value, the Company performs a hypothetical purchase price allocation based on the reporting unit’s fair value to determine the fair value of the reporting unit’s goodwill. Any resulting difference will be a charge to Amortization and impairment of intangible assets in the Consolidated Statements of Income in the period in which the determination is made. Fair value is determined using a combination of present value techniques and market prices of comparable businesses. We classify our intangible assets acquired as either tradenames, customer-related and contract-based, or technology and other. Amortization basis and estimated useful lives by intangible asset type are generally as follows: Intangible Asset Description Amortization Basis Estimated Useful Life Tradenames Straight-line 1 to 3 years Customer-related and contract-based In line with underlying cash flows 7 to 20 years Technology and other Straight-line 5 to 7 years
Derivatives	Derivatives Derivative instruments are recognized in the Consolidated Statements of Financial Position at fair value. Where the Company has entered into master netting agreements with counterparties, the derivative positions are netted by counterparties and are reported accordingly in other assets or other liabilities. Changes in the fair value of derivative instruments are recognized in earnings each period, unless the derivative is designated and qualifies as a cash flow or net investment hedge. The Company has historically designated the following hedging relationships for certain transactions: (1) a hedge of the change in fair value of a recognized asset or liability or firm commitment (“fair value hedge”), (2) a hedge of the variability in cash flows from a recognized variable-rate asset or liability or forecasted transaction (“cash flow hedge”), and (3) a hedge of the net investment in a foreign operation (“net investment hedge”). In order for a derivative to qualify for hedge accounting, the derivative must be formally designated as a fair value, cash flow, or a net investment hedge by documenting the relationship between the derivative and the hedged item. The documentation must include a description of the hedging instrument, the hedged item, the risk being hedged, Aon’s risk management objective and strategy for undertaking the hedge, and the method for assessing the effectiveness of the hedge. Additionally, the hedge relationship must be expected to be highly effective at offsetting changes in either the fair value or cash flows of the hedged item at both the inception of the hedge and on an ongoing basis. Aon assesses the ongoing effectiveness of its hedges quarterly or more frequently if facts and circumstances require. For a derivative designated as a fair value hedging instrument, the gain or loss is recognized in earnings in the period of change together with the offsetting loss or gain on the hedged item attributable to the risk being hedged. The effect is to reflect in earnings the extent to which the hedge is not effective in achieving offsetting changes in fair value. For a cash flow hedge that qualifies for hedge accounting, the change in fair value of a hedging instrument is recognized in Accumulated other comprehensive income (“AOCI”) and subsequently reclassified to earnings in the same period the hedged item impacts earnings. For a net investment hedge, the change in fair value of the hedging instrument is recognized in AOCI as part of the cumulative translation adjustment. Changes in the fair value of a derivative that is not designated as part of a hedging relationship (commonly referred to as an “economic hedge”) are recorded in Other income (expense) in the Consolidated Statements of Income in the period of change. The Company discontinues hedge accounting prospectively when (1) the derivative expires or is sold, terminated, or exercised, (2) the qualifying criteria are no longer met, or (3) management removes the designation of the hedging relationship.
Foreign Currency	Foreign Currency Certain of the Company’s non-U.S. operations use their respective local currency as their functional currency. These operations that do not have the U.S. dollar as their functional currency translate their financial statements at the current rates of exchange in effect at the balance sheet date and revenues and expenses using rates that approximate those in effect during the period. The resulting translation adjustments are included in Net foreign currency translation adjustments within the Consolidated Statements of Shareholders’ Equity. Further, gains and losses from the remeasurement of monetary assets and liabilities that are denominated in a non-functional currency of that entity are included in Other income (expense)
Income Taxes	Income Taxes Deferred income taxes are recognized for the effect of temporary differences between financial reporting and tax basis of assets and liabilities and are measured using the enacted marginal tax rates and laws that are currently in effect. The effect on deferred tax assets and liabilities from a change in tax rates is recognized in the period when the rate change is enacted. Deferred tax assets are reduced by valuation allowances if, based on the consideration of all available evidence, it is more likely than not that some portion of the deferred tax asset will not be realized. Deferred tax assets are realized by having sufficient future taxable income to allow the related tax benefits to reduce taxes otherwise payable. The sources of taxable income that may be available to realize the benefit of deferred tax assets are future reversals of existing taxable temporary differences, future taxable income exclusive of reversing temporary differences and carry-forwards, taxable income in carry-back years, and tax planning strategies that are both prudent and feasible. The Company recognizes the effect of income tax positions only if sustaining those positions is more likely than not. Tax positions that meet the more likely than not recognition threshold but are not highly certain are initially and subsequently measured based on the largest amount of benefit that is greater than 50% likely of being realized upon settlement with the taxing authority. Only information that is available at the reporting date is considered in the Company’s recognition and measurement analysis, and events or changes in facts and circumstances are accounted for in the period in which the event or change in circumstance occurs. The Company records penalties and interest related to unrecognized tax benefits in Income taxes in the Company’s Consolidated Statements of Income.
Leases	Leases The Company leases office facilities, equipment, and automobiles under non-cancelable operating and finance leases. The Company’s lease obligations are primarily for the use of office facilities. The Company evaluates if a leasing arrangement exists upon inception of a contract. A contract contains a lease if the contract conveys the right to control the use of identified tangible assets for a period of time in exchange for consideration. Identified property, plant, or equipment may include a physically distinct portion of a larger asset, or a portion of an asset that represents substantially all of the capacity of the asset but is not physically distinct. The Company assesses whether a contract implicitly contains the right to control the use of a tangible asset that is not already owned. In addition, the Company subleases certain real estate properties to third parties, which are classified as operating leases. The Company’s leases expire at various dates and may contain renewal and expansion options. The exercise of lease renewal and expansion options are typically at the Company’s sole discretion and are only included in the determination of the lease term if the Company is reasonably certain to exercise the option. The Company’s leases do not typically contain termination options. In addition, the Company’s lease agreements typically do not contain any material residual value guarantees or restrictive covenants. ROU assets and lease liabilities are based on the present value of the minimum lease payments over the lease term. The Company has elected the practical expedient related to lease and non-lease components, as an accounting policy election for all asset classes, which allows a lessee to not separate non-lease from lease components and instead account for consideration received in a contract as a single lease component. The Company made a policy election to not recognize ROU assets and lease liabilities that arise from leases with an initial term of twelve months or less on the Consolidated Statements of Financial Position. However, the Company recognized these lease payments in the Consolidated Statements of Income on a straight-line basis over the lease term and variable lease payments in the period in which the expense was incurred. The Company chose to apply this accounting policy across all classes of underlying assets. A portion of the Company’s lease agreements include variable lease payments that are not recorded in the initial measurement of the lease liability and ROU asset balances. For real estate arrangements, base rental payments may be escalated according to annual changes in the Consumer Price Index (“CPI”) or other indices. The escalated rental payments based on the estimated CPI at the lease commencement date are included within minimum rental payments; however, changes in CPI are considered variable in nature and are recognized as variable lease costs in the period in which the obligation is incurred. Additionally, real estate lease agreements may include other variable payments related to operating expenses charged by the landlord based on actual expenditures. Information technology equipment agreements may include variable payments based on usage of the equipment. These expenses are also recognized as variable lease costs in the period in which the expense is incurred. The Company utilizes discount rates to determine the present value of the lease payments based on information available at the commencement date of the lease. As the rate implicit in each lease is not typically readily available, the Company uses an incremental borrowing rate based on factors such as the lease term and the economic environment where the lease exists to determine the appropriate present value of future lease payments. When determining the incremental borrowing rate, the Company considers the rate of interest it would pay on a secured borrowing in an amount equal to the lease payments for the underlying asset under similar terms. Operating leases are included in Operating lease ROU assets, Other current liabilities, and Non-current operating lease liabilities on the Consolidated Statements of Financial Position. Finance leases are included in Other non-current assets, Other current liabilities, and Other non-current liabilities on the Consolidated Statements of Financial Position.
Principles of Consolidation	Principles of Consolidation
New Accounting Pronouncements	New Accounting Pronouncements Adoption of New Accounting Standards Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the Financial Accounting Standards Board (“FASB”) issued new accounting guidance related to reclassification of certain tax effects from accumulated other comprehensive income. The guidance allowed a reclassification from accumulated comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act of 2017. The guidance was effective for the Company in the first quarter of 2019. Aon did not elect to reclassify stranded tax effects on the Consolidated Statement of Financial Position; therefore, for the three and twelve months ended December 31, 2019 , there was no impact on the net income of the Company. It is the Company’s policy to release income tax effects from accumulated other comprehensive loss using the portfolio approach. Targeted Improvements to Accounting for Hedging Activities In August 2017, the FASB issued new accounting guidance related to targeted improvements to accounting for hedging activities. The new guidance amended its hedge accounting model to enable entities to better portray their risk management activities in the financial statements. The guidance eliminated the requirement to separately measure and report hedge ineffectiveness and required the effect of a hedging instrument to be presented in the same income statement line as the hedged item. The new guidance was effective for Aon in the first quarter of 2019 and the Company adopted it on a modified retrospective basis with no cumulative effect adjustment to Accumulated other comprehensive income (loss) or corresponding adjustment to Retained earnings. Changes to the Consolidated Statement of Income and financial statement disclosures were applied prospectively. Under the new guidance, gains or losses on certain derivative hedging instruments are recognized in Revenue, as opposed to Other income (expense) under the previous guidance. For the three and twelve months ended December 31, 2019 , the adoption of this guidance had no impact on the net income and an insignificant impact on the operating income of the Company. Leases In February 2016, the FASB issued a new accounting standard related to leases, which required lessees to recognize assets and liabilities for most leases. Under the new standard, a lessee is required to recognize in the Consolidated Statements of Financial Position, liabilities to make future lease payments and ROU assets representing its right to use the underlying assets for the lease term. The recognition, measurement, timing, and presentation of expenses and cash flows arising from a lease by a lessee did not significantly changed from previous U.S. GAAP. Under previous U.S. GAAP, leases classified as operating were not required to be recognized as assets and liabilities on the Consolidated Statements of Financial Position, and expense for minimum lease payments would be recognized on a straight-line basis over the term of the lease in the Consolidated Statements of Income. Leases classified as capital leases under previous U.S. GAAP were initially recorded using the present value of the minimum lease payments, with an associated capital lease liability, classified as short-term or long-term, as appropriate in the Consolidated Statements of Financial Position. The Company adopted the new standard as of January 1, 2019 using the modified retrospective approach for all leases existing at, or entered into after, the period of adoption. Under this approach, prior periods were not restated. Rather, lease balances and other disclosures for prior periods were provided in the Notes to Consolidate Financial Statements as previously reported, and the cumulative effect of initially applying the guidance was recognized in the Consolidated Statement of Financial Position as of December 31, 2019 . The modified retrospective approach included several optional practical expedients available that entities could elect to apply upon transition. These practical expedients related to the identification and classification of leases that commenced before the effective date, initial direct costs for leases that commenced before the effective date, and the ability to use hindsight in evaluating lessee options to extend or terminate a lease or to purchase the underlying asset. The Company elected the package of practical expedients permitted under the transition guidance within the new standard, which allowed a lessee to carry forward their population of existing leases, the classification of each lease, as well as the treatment of initial direct costs as of the period of adoption. In addition, the Company elected the practical expedient related to lease and non-lease components, as an accounting policy election for all asset classes, which allowed a lessee to not separate non-lease from lease components and instead account for consideration paid in a contract as a single lease component. Lastly, the Company did not elect the practical expedient related to hindsight analysis which allowed a lessee to use hindsight in determining the lease term and in assessing impairment of the entity’s ROU assets. The Company made a policy election to not recognize ROU assets and lease liabilities that arise from leases with an initial term of twelve months or less on the Consolidated Statement of Financial Position. However, the Company recognized these lease payments in the Consolidated Statement of Income on a straight-line basis over the lease term and variable lease payments in the period in which the expense was incurred. The Company chose to apply this accounting policy across all classes of underlying assets. Additionally, upon adoption, the Company utilized a discount rate to determine the present value of the lease payments based on information available as of January 1, 2019. Beginning January 1, 2019, operating ROU assets and operating lease liabilities were recognized based on the present value of lease payments over the lease term at the commencement date. Operating leases in effect prior to January 1, 2019 were recognized at the present value of the remaining payments for the remaining lease term as of January 1, 2019. Upon adoption, the Company recognized ROU assets and lease liabilities of $1.1 billion and $1.3 billion , respectively. The standard had an insignificant impact on the Consolidated Statement of Income and no impact on the Consolidated Statement of Cash Flows. Refer to Note 10 “Lease Commitments” for further information including significant assumptions and judgments made. As a result of applying the modified retrospective approach to adopt the new leasing standard, the following adjustments were made to the Consolidated Statement of Financial Position as of January 1, 2019 (in millions): December 31, January 1, As Reported Adjustments As Adjusted Assets Operating lease right-of-use assets $ — $ 1,021 $ 1,021 Other non-current assets $ 448 $ 78 $ 526 Liabilities Other current liabilities $ 936 $ 219 $ 1,155 Non-current operating lease liabilities $ — $ 1,014 $ 1,014 Other non-current liabilities $ 1,097 $ (134 ) $ 963 Revenue Recognition In May 2014, the FASB issued a new accounting standard on revenue from contracts with customers (the “Standard” or “ASC 606”), which superseded nearly all existing revenue recognition guidance under U.S. GAAP (“ASC 605”). The core principal of the Standard is that an entity should recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The Company elected to apply the modified retrospective adoption approach to all contracts. Under this approach, prior periods were not restated. Rather, revenues and other disclosures for prior periods were provided in the notes to the Consolidated Financial Statements as previous reported under ASC 605, and the cumulative effect of initially applying the standard was recognized as an adjustment to Retained earnings for approximately $507 million . The following summarizes the significant changes to the Company as a result of the adoption of ASC 606 on January 1, 2018. • The Company previously recognized revenue either at a point in time or over a period of time based on the transfer of value to customers or as the remuneration became determinable. Under ASC 606, the revenue related to certain brokerage services recognized over a period of time is recognized on the effective date of the associated policies when control of the policy transfers to the customer. This change resulted in a significant shift in timing of interim revenue for the Reinsurance Solutions revenue line and, to a lesser extent, certain other brokerage services. • The Standard provides guidance on accounting for certain revenue-related costs including when to capitalize costs associated with obtaining and fulfilling a contract. The majority of these costs were previously expensed as incurred under ASC 605. Assets recognized for the costs to obtain a contract, which includes certain sales commissions, are amortized on a systematic basis that is consistent with the transfer of the services to which the asset relates, considering anticipated renewals when applicable. Assets recognized as costs to fulfill a contract, which includes internal costs related to pre-placement broking activities, as well as other costs, are amortized on a systematic basis that is consistent with the transfer of the services to which the asset relates, which is generally less than one year. Accounting Standards Issued but Not Yet Adopted Simplifying the Accounting for Income Taxes In December 2019, the FASB issued new accounting guidance that simplifies the accounting for income taxes by eliminating some exceptions to the general approach in the existing guidance. It also clarifies certain aspects of the existing guidance to promote more consistent application. The new guidance is effective for Aon in the first quarter of 2021, with early adoption permitted. The Company is currently evaluating the impact that the guidance will have on the Consolidated Financial Statements and the period of adoption. Cloud Computing Arrangements In August 2018, the FASB issued new accounting guidance on implementation costs incurred in a cloud computing arrangement that is a service contract. The new guidance aligns capitalization requirements for certain implementation costs incurred in cloud computing arrangements with existing requirements for capitalizing implementation costs for internal-use software. These costs will be deferred over the term of the hosting arrangement, including any optional renewal periods the entity is reasonably certain to exercise. An entity may apply the new guidance on either a prospective or retrospective basis. The new guidance is effective for Aon in the first quarter of 2020. The Company will adopt the new guidance on a prospective basis for all implementation costs incurred after the date of initial adoption. The Company does not expect adoption to have a significant impact on the Consolidated Financial Statements. Changes to the Disclosure Requirements for Defined Benefit Plans In August 2018, the FASB issued new accounting guidance related to the disclosure requirements for employers that sponsor defined benefit pension and other postretirement benefit plans. The guidance requires sponsors of these plans to provide additional disclosures, including weighted average interest rates used in the entity’s cash balance pension plans and a narrative description of reasons for any significant gains or losses impacting the benefit obligation for the period, and eliminates certain previous disclosure requirements. The new guidance is effective for Aon in the first quarter of 2021 with early adoption permitted and will be applied retrospectively. The Company is currently evaluating the impact that the guidance will have on the Consolidated Financial Statements and the period of adoption. Simplifying the Test for Goodwill Impairment In January 2017, the FASB issued new accounting guidance on simplifying the test for goodwill impairment. Currently the standard requires an entity to perform a two-step test to determine the amount, if any, of goodwill impairment. In Step 1, an entity compares the fair value of a reporting unit with its carrying amount, including goodwill. If the carrying amount of the reporting unit exceeds its fair value, the entity performs Step 2 and compares the implied fair value of goodwill with the carrying amount of that goodwill for that reporting unit. An impairment charge equal to the amount by which the carrying amount of goodwill for the reporting unit exceeds the implied fair value of that goodwill is recorded, limited to the amount of goodwill allocated to that reporting unit. The new guidance removes Step 2. An entity will apply a one-step quantitative test and record the amount of goodwill impairment as the excess of a reporting unit’s carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The new guidance does not amend the optional qualitative assessment of goodwill impairment. The Company will adopt the new guidance in the first quarter of 2020 on a prospective basis. The Company does not expect adoption to have a significant impact on the Consolidated Financial Statements. Credit Losses
Fair Value Measurements	Accounting standards establish a three tier fair value hierarchy that prioritizes the inputs used in measuring fair values as follows: • Level 1 — observable inputs such as quoted prices for identical assets in active markets; • Level 2 — inputs other than quoted prices for identical assets in active markets, that are observable either directly or indirectly; and • Level 3 — unobservable inputs in which there is little or no market data which requires the use of valuation techniques and the development of assumptions. The following methods and assumptions are used to estimate the fair values of the Company’s financial instruments, including pension assets (refer to Note 13 “Employee Benefits”): Money market funds consist of institutional prime, treasury, and government money market funds. The Company reviews treasury and government money market funds to obtain reasonable assurance that the fund net asset value is $1 per share, and reviews the floating net asset value of institutional prime money market funds for reasonableness. Cash and cash equivalents consist of cash and institutional short-term investment funds. The Company reviews the short-term investment funds to obtain reasonable assurance that the fund net asset value is $1 per share. Equity investments consist of equity securities and equity derivatives valued using the closing stock price on a national securities exchange. Over the counter equity derivatives are valued using observable inputs such as underlying prices of the underlying security and volatility. On a sample basis the Company reviews the listing of Level 1 equity securities in the portfolio and agrees the closing stock prices to a national securities exchange, and independently verifies the observable inputs for Level 2 equity derivatives and securities. Fixed income investments consist of certain categories of bonds and derivatives. Corporate, government, and agency bonds are valued by pricing vendors who estimate fair value using recently executed transactions and proprietary models based on observable inputs, such as interest rate spreads, yield curves, and credit risk. Asset-backed securities are valued by pricing vendors who estimate fair value using discounted cash flow models utilizing observable inputs based on trade and quote activity of securities with similar features. Fixed income derivatives are valued by pricing vendors using observable inputs such as interest rates and yield curves. The Company obtains an understanding of the models, inputs, and assumptions used in developing prices provided by its vendors through discussions with the fund managers. The Company independently verifies the observable inputs, as well as assesses assumptions used for reasonableness based on relevant market conditions and internal Company guidelines. If an assumption is deemed unreasonable, based on the Company’s guidelines, it is then reviewed by management and the fair value estimate provided by the vendor is adjusted, if deemed appropriate. These adjustments do not occur frequently and historically are not material to the fair value estimates used in the Consolidated Financial Statements. Pooled funds consist of various equity, fixed income, commodity, and real estate mutual fund type investment vehicles. Pooled investment funds fair value is estimated based on the proportionate share ownership in the underlying net assets of the investment, which is based on the fair value of the underlying securities that trade on a national securities exchange. The Company gains an understanding of the investment guidelines and valuation policies of the fund and discusses fund performance with pooled fund managers. The Company obtains audited fund manager financial statements, when available. If the pooled fund is designed to replicate a publicly traded index, the Company compares the performance of the fund to the index to assess the reasonableness of the fair value measurement. Alternative investments consist of limited partnerships, private equity, and hedge funds. Alternative investment fair value is generally estimated based on the proportionate share ownership in the underlying net assets of the investment as determined by the general partner or investment manager. The valuations are based on various factors depending on investment strategy, proprietary models, and specific financial data or projections. The Company obtains audited fund manager financial statements, when available. The Company obtains a detailed understanding of the models, inputs, and assumptions used in developing prices provided by the investment managers, or appropriate party, through regular discussions. The Company also obtains the investment manger’s valuation policies and assesses the assumptions used for reasonableness based on relevant market conditions and internal Company guidelines. If an assumption is deemed unreasonable, based on the Company’s guidelines, it is then reviewed by management and the fair value estimate provided by the vendor is adjusted, if deemed appropriate. These adjustments do not occur frequently and historically are not material to the fair value estimates in the Consolidated Financial Statements. Derivatives are carried at fair value, based upon industry standard valuation techniques that use, where possible, current market-based or independently sourced pricing inputs, such as interest rates, currency exchange rates, or implied volatilities. Annuity contracts consist of insurance group annuity contracts purchased to match the pension benefit payment stream owed to certain selected plan participant demographics within a few major U.K. defined benefit plans. Annuity contracts are valued using a discounted cash flow model utilizing assumptions such as discount rate, mortality, and inflation. Real estate and REITs consist of publicly traded REITs and direct real estate investments. Level 1 REITs are valued using the closing stock price on a national securities exchange. Non-Level 1 values are based on the proportionate share of ownership in the underlying net asset value as determined by the investment manager. The Company independently reviews the listing of Level 1 REIT securities in the portfolio and agrees the closing stock prices to a national securities exchange. The Company gains an understanding of the investment guidelines and valuation policies of the non-Level 1 real estate funds and discusses performance with the fund managers. The Company obtains audited fund manager financial statements, when available. See the description of “Alternative investments” for further detail on valuation procedures surrounding non-Level 1 REITs. Debt is carried at outstanding principal balance, less any unamortized issuance costs, discount or premium. Fair value is based on quoted market prices or estimates using discounted cash flow analyses based on current borrowing rates for similar types of borrowing arrangements.

Summary of Significant Accoun_3

Summary of Significant Accounting Principles and Practices (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Schedule of estimated useful lives of assets	Depreciation and amortization are computed using the straight-line method over the estimated useful lives of the assets, which are generally as follows: Asset Description Estimated Useful Life Software Lesser of the life of an associated license, or 4 to 7 years Leasehold improvements Lesser of estimated useful life or lease term, not to exceed 10 years Furniture, fixtures and equipment 4 to 10 years Computer equipment 4 to 6 years Buildings 35 years Automobiles 6 years
Schedule of other intangible assets by asset class	Amortization basis and estimated useful lives by intangible asset type are generally as follows: Intangible Asset Description Amortization Basis Estimated Useful Life Tradenames Straight-line 1 to 3 years Customer-related and contract-based In line with underlying cash flows 7 to 20 years Technology and other Straight-line 5 to 7 years Other intangible assets by asset class are as follows (in millions): As of December 31 2019 2018 Gross Carrying Amount Accumulated Amortization and Impairment Net Carrying Amount Gross Carrying Amount Accumulated Amortization and Impairment Net Carrying Amount Customer-related and contract-based $ 2,264 $ 1,600 $ 664 $ 2,240 $ 1,444 $ 796 Tradenames 1,029 956 73 1,027 740 287 Technology and other 380 334 46 391 325 66 Total $ 3,673 $ 2,890 $ 783 $ 3,658 $ 2,509 $ 1,149
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	As a result of applying the modified retrospective approach to adopt the new leasing standard, the following adjustments were made to the Consolidated Statement of Financial Position as of January 1, 2019 (in millions): December 31, January 1, As Reported Adjustments As Adjusted Assets Operating lease right-of-use assets $ — $ 1,021 $ 1,021 Other non-current assets $ 448 $ 78 $ 526 Liabilities Other current liabilities $ 936 $ 219 $ 1,155 Non-current operating lease liabilities $ — $ 1,014 $ 1,014 Other non-current liabilities $ 1,097 $ (134 ) $ 963

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	The following table summarizes revenue from contracts with customers by principal service line (in millions): Years ended December 31 2019 2018 2017 Commercial Risk Solutions $ 4,673 $ 4,652 $ 4,169 Reinsurance Solutions 1,686 1,563 1,429 Retirement Solutions 1,817 1,865 1,755 Health Solutions 1,667 1,596 1,515 Data & Analytic Services 1,184 1,105 1,140 Elimination (14 ) (11 ) (10 ) Total revenue $ 11,013 $ 10,770 $ 9,998 Consolidated revenue from contracts with customers by geographic area, which is attributed on the basis of where the services are performed, is as follows (in millions): Years ended December 31 2019 2018 2017 United States $ 5,016 $ 4,677 $ 4,425 Americas other than United States 919 940 976 United Kingdom 1,502 1,555 1,436 Europe, Middle East, & Africa other than United Kingdom 2,338 2,413 2,025 Asia Pacific 1,238 1,185 1,136 Total revenue $ 11,013 $ 10,770 $ 9,998
Capitalized Contract Cost	Changes in the net carrying amount of costs to obtain contracts with customers are as follows (in millions): 2019 2018 Balance at beginning of period $ 156 $ 145 Additions 58 53 Amortization (44 ) (41 ) Impairment — — Foreign currency translation and other 1 (1 ) Balance at end of period $ 171 $ 156 Changes in the net carrying amount of costs to fulfill contracts with customers are as follows (in millions): 2019 2018 Balance at beginning of period $ 329 $ 298 Additions 1,453 1,504 Amortization (1,447 ) (1,465 ) Impairment — — Foreign currency translation and other — (8 ) Balance at end of period $ 335 $ 329

Other Financial Data (Tables)

Other Financial Data (Tables)	12 Months Ended
	Dec. 31, 2019
Other Financial Data [Abstract]
Other income (Expense)	The components of Other income (expense) are as follows (in millions): Years ended December 31 2019 2018 2017 Foreign currency remeasurement $ 9 $ 25 $ (37 ) Disposal of businesses 13 (6 ) (16 ) Pension and other postretirement 9 1 (86 ) Equity earnings 4 4 12 Financial instruments (34 ) (49 ) 2 Total $ 1 $ (25 ) $ (125 )
Schedule of Allowance for Doubtful Accounts	Changes in the net carrying amount of allowance for doubtful accounts are as follows (in millions): 2019 2018 2017 Balance at beginning of period $ 62 $ 59 $ 56 Provision 27 24 18 Accounts written off, net of recoveries (19 ) (25 ) (18 ) Foreign currency translation and other — 4 3 Balance at end of period $ 70 $ 62 $ 59
Schedule of Other Current Assets	The components of Other current assets are as follows (in millions): As of December 31 2019 2018 Costs to fulfill contracts with customers $ 335 $ 329 Taxes receivable 88 113 Prepaid expenses 97 97 Receivables from the Divested Business (1) 4 12 Other 78 67 Total $ 602 $ 618 (1) Refer to Note 5 “Discontinued Operations” for further information.
Components of Fixed assets, net	The components of Fixed assets, net are as follows (in millions): As of December 31 2019 2018 Software $ 727 $ 693 Leasehold improvements 358 334 Computer equipment 250 279 Furniture, fixtures and equipment 225 228 Construction in progress 183 154 Other 37 45 Fixed assets, gross 1,780 1,733 Less: Accumulated depreciation 1,159 1,145 Fixed assets, net $ 621 $ 588
Schedule of Other Non-current Assets	The components of Other non-current assets are as follows (in millions): As of December 31 2019 2018 Costs to obtain contracts with customers $ 171 $ 156 Investments 53 54 Leases (1) 100 — Taxes receivable 102 100 Other 144 138 Total $ 570 $ 448 (1)
Schedule of Other Current Liabilities	The components of Other current liabilities are as follows (in millions): As of December 31 2019 2018 Deferred revenue (1) $ 270 $ 251 Taxes payable 93 83 Leases (2) 210 — Other 513 602 Total $ 1,086 $ 936 (1) $532 million and $487 million was recognized in the Consolidated Statements of Income during the twelve months ended December 31, 2019 and December 31, 2018 , respectively. (2) In the first quarter of 2019, Aon adopted new accounting guidance related to the treatment of leases which was applied under the modified retrospective approach. The Other current liability balances related to leasing under the legacy accounting standard are reflected in Other as reported in December 31, 2018. Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for further information.
Schedule of Other Non-current Liabilities	The components of Other non-current liabilities are as follows (in millions): As of December 31 2019 2018 Taxes payable (1) $ 525 $ 585 Leases (2) 76 169 Compensation and benefits 49 56 Deferred revenue 62 65 Other 165 222 Total $ 877 $ 1,097 (1) Includes $145 million and $240 million for the non-current portion of the Transition Tax, as of December 31, 2019 and December 31, 2018 , respectively. Refer to Note 11 “Income Taxes” for further information on the Transition Tax. (2) In the first quarter of 2019, Aon adopted new accounting guidance related to the treatment of leases which was applied under the modified retrospective approach. The comparable period presented reflects balances under the legacy accounting standard. Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for further information.

Discontinued Operations (Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2019
Discontinued Operations and Disposal Groups [Abstract]
Schedule of Discontinued Operations	The following table presents the financial results of the Divested Business (in millions): Years ended December 31 2019 2018 2017 Revenue Total revenue $ — $ — $ 698 Expenses Total operating expenses 2 12 656 Operating income from discontinued operations (2 ) (12 ) 42 Other income — — 10 Income (loss) from discontinued operations before income taxes (2 ) (12 ) 52 Income tax expense (benefit) (1 ) (4 ) 3 Net income (loss) from discontinued operations, excluding gain (1 ) (8 ) 49 Gain on sale of discontinued operations, net of tax — 82 779 Net income (loss) from discontinued operations $ (1 ) $ 74 $ 828

Restructuring Restructuring and

Restructuring Restructuring and Related Activities (Tables)	12 Months Ended
	Dec. 31, 2019
Restructuring and Related Activities [Abstract]
Restructuring and Related Costs	The following table summarizes restructuring and related expenses by type that were incurred through the end of the Restructuring Plan (in millions): 2019 2018 2017 Completed Plan Total Workforce reduction $ 205 $ 115 $ 299 $ 619 Technology rationalization (1) 39 47 33 119 Lease consolidation (1) 33 28 8 69 Asset impairments 14 13 26 53 Other costs associated with restructuring and separation (1) (2) 160 282 131 573 Total restructuring and related expenses $ 451 $ 485 $ 497 $ 1,433 (1) Total contract termination costs incurred under the Restructuring Plan associated with technology rationalizations, lease consolidations, and other costs associated with restructuring and separation for the twelve months ended December 31, 2019 were $0 million , $33 million , and $13 million , respectively; for the twelve months ended December 31, 2018 were $5 million , $25 million , and $85 million , respectively; and for the twelve months ended December 31, 2017 were $1 million , $8 million , and $3 million , respectively. (2) Other costs associated with the Restructuring Plan include those to separate the Divested Business, as well as moving costs, and consulting and legal fees. These costs are typically recognized when incurred.
Schedule of Restructuring Reserve by Type of Cost	The changes in the Company’s liabilities for the Restructuring Plan as of December 31, 2019 are as follows (in millions): Restructuring Plan Balance at December 31, 2018 $ 201 Expensed 418 Cash payments (415 ) Foreign currency translation and other — Balance at December 31, 2019 $ 204

Acquisitions and Dispositions_2

Acquisitions and Dispositions of Businesses (Tables)	12 Months Ended
	Dec. 31, 2019
Business Combinations and Discontinued Operations and Disposal Groups [Abstract]
Consideration transferred and preliminary value of intangible assets	The following table includes the preliminary fair values of consideration transferred, assets acquired, and liabilities assumed as a result of the Company’s acquisitions (in millions): For the year ended December 31, 2019 Consideration transferred Cash $ 42 Deferred and contingent consideration 8 Aggregate consideration transferred $ 50 Assets acquired Cash and cash equivalents $ 4 Receivables, net — Goodwill 34 Intangible assets, net 22 Fixed assets, net 1 Other assets 13 Total assets acquired 74 Liabilities assumed Current liabilities 18 Other non-current liabilities 6 Total liabilities assumed 24 Net assets acquired $ 50

Goodwill and Other Intangible_2

Goodwill and Other Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of changes in the net carrying amount of goodwill by operating segment	The changes in the net carrying amount of goodwill for the years ended December 31, 2019 and 2018 , respectively, are as follows (in millions): Balance as of January 1, 2018 $ 8,358 Goodwill related to current year acquisitions 38 Goodwill related to disposals (2 ) Goodwill related to prior year acquisitions 4 Foreign currency translation (227 ) Balance as of December 31, 2018 $ 8,171 Goodwill related to current year acquisitions 34 Goodwill related to disposals (11 ) Goodwill related to prior year acquisitions 2 Foreign currency translation (31 ) Balance as of December 31, 2019 $ 8,165
Schedule of other intangible assets by asset class	Amortization basis and estimated useful lives by intangible asset type are generally as follows: Intangible Asset Description Amortization Basis Estimated Useful Life Tradenames Straight-line 1 to 3 years Customer-related and contract-based In line with underlying cash flows 7 to 20 years Technology and other Straight-line 5 to 7 years Other intangible assets by asset class are as follows (in millions): As of December 31 2019 2018 Gross Carrying Amount Accumulated Amortization and Impairment Net Carrying Amount Gross Carrying Amount Accumulated Amortization and Impairment Net Carrying Amount Customer-related and contract-based $ 2,264 $ 1,600 $ 664 $ 2,240 $ 1,444 $ 796 Tradenames 1,029 956 73 1,027 740 287 Technology and other 380 334 46 391 325 66 Total $ 3,673 $ 2,890 $ 783 $ 3,658 $ 2,509 $ 1,149
Schedule of estimated future amortization expense on intangible assets	The estimated future amortization for finite-lived intangible assets as of December 31, 2019 is as follows (in millions): Estimated Future Amortization For the years ended 2020 $ 223 2021 127 2022 86 2023 76 2024 59 Thereafter 212 Total $ 783

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Summary of outstanding debt	The following is a summary of outstanding debt (in millions): As of December 31 2019 2018 Commercial paper $ 112 $ 250 5.00% Senior Notes due September 2020 600 599 2.80% Senior Notes due March 2021 399 398 2.20% Senior Notes due November 2022 497 — 4.00% Senior Notes due November 2023 348 348 3.50% Senior Notes due June 2024 597 596 3.875% Senior Notes due December 2025 746 746 2.875% Senior Notes due May 2026 (EUR 500M) 550 562 8.205% Junior Subordinated Notes due January 2027 521 521 4.50% Senior Notes due December 2028 346 347 3.75% Senior Notes due May 2029 744 — 6.25% Senior Notes due September 2040 296 296 4.25% Senior Notes due December 2042 199 198 4.45% Senior Notes due May 2043 246 246 4.60% Senior Notes due June 2044 544 544 4.75% Senior Notes due May 2045 593 592 Other 1 1 Total debt 7,339 6,244 Less: Short-term debt and current portion of long-term debt 712 251 Total long-term debt $ 6,627 $ 5,993
Repayments of long-term debt	Repayments of total debt are as follows (in millions): 2020 $ 712 2021 400 2022 500 2023 350 2024 600 Thereafter 4,882 Total Repayments 7,444 Unamortized discounts, premiums, and debt issuance costs (105 ) Total Debt $ 7,339
Schedule of Commercial Paper	Commercial paper outstanding, which is included in Short-term debt and current portion of long-term debt in the Company’s Consolidated Statements of Financial Position, is as follows (in millions): As of December 31 2019 2018 Commercial paper outstanding $ 112 $ 250 The weighted average commercial paper outstanding and its related interest rates are as follows (in millions, except percentages): Years ended December 31 2019 2018 Weighted average commercial paper outstanding $ 511 $ 580 Weighted average interest rate of commercial paper outstanding 0.27 % 0.84 %

Lease Commitments (Tables)

Lease Commitments (Tables)	12 Months Ended
	Dec. 31, 2019
Leases, Operating [Abstract]
Assets And Liabilities Of Lessee	The classification of operating and finance lease asset and liability balances within the Consolidated Statement of Financial Position is as follows (in millions): As of December 31, 2019 Assets Operating lease assets Operating lease right-of-use assets $ 929 Finance lease assets Other non-current assets 100 Total lease assets $ 1,029 Liabilities Current lease liabilities Operating Other current liabilities $ 186 Finance Other current liabilities 24 Non-current lease liabilities Operating Non-current operating lease liabilities 944 Finance Other non-current liabilities 76 Total lease liabilities $ 1,230
Lease, Cost	Weighted average remaining lease term and discount rate related to operating and finance leases are as follows: As of December 31, 2019 Weighted average remaining lease term (years) Operating leases 7.9 Finance leases 4.4 Weighted average discount rate Operating leases 3.2 % Finance leases 2.0 % Other cash and non-cash related activities are as follows (in millions): Year Ended December 31, 2019 Cash paid for amounts included in the measurement of lease liabilities Operating cash flows for operating leases $ 264 Financing cash flows for finance leases $ 17 Non-cash related activities ROU assets obtained in exchange for new operating lease liabilities $ 155 ROU assets obtained in exchange for new finance lease liabilities $ 48 Operating lease ROU asset expense (1) $ 195 Changes in Non-current operating lease liabilities (1) $ (70 ) (1) The Company has recorded non-cash changes in Operating lease ROU assets and Non-current operating lease liabilities through Other assets and liabilities in Cash flows from operations within the Consolidated Statement of Cash Flows. The components of lease costs are as follows (in millions): Year Ended December 31, 2019 Operating lease cost $ 234 Finance lease costs Amortization of leased assets 26 Interest on lease liabilities 2 Variable lease cost 60 Short-term lease cost (1) 5 Sublease income (32 ) Net lease cost $ 295 (1) Short-term lease cost does not include expenses related to leases with a lease term of one month or less.
Lessee Operating Lease Liability Maturity	Maturity analysis of operating and finance leases as of December 31, 2019 are as follows (in millions): Operating Leases Finance Leases Total 2020 $ 208 $ 26 $ 234 2021 202 26 228 2022 177 20 197 2023 141 18 159 2024 113 15 128 Thereafter 432 — 432 Total undiscounted future minimum lease payments 1,273 105 1,378 Less: Imputed interest (143 ) (5 ) (148 ) Present value of lease liabilities $ 1,130 $ 100 $ 1,230 At December 31, 2018, future minimum rental payments required under operating leases that have initial or remaining non-cancelable lease terms in excess of one year are as follows (in millions): Gross rental commitments Rentals from subleases Net rental commitments 2019 $ 303 $ (34 ) $ 269 2020 253 (30 ) 223 2021 221 (30 ) 191 2022 182 (30 ) 152 2023 148 (12 ) 136 Thereafter 472 (5 ) 467 Total minimum payments required $ 1,579 $ (141 ) $ 1,438
Finance Lease, Liability, Maturity	Maturity analysis of operating and finance leases as of December 31, 2019 are as follows (in millions): Operating Leases Finance Leases Total 2020 $ 208 $ 26 $ 234 2021 202 26 228 2022 177 20 197 2023 141 18 159 2024 113 15 128 Thereafter 432 — 432 Total undiscounted future minimum lease payments 1,273 105 1,378 Less: Imputed interest (143 ) (5 ) (148 ) Present value of lease liabilities $ 1,130 $ 100 $ 1,230 At December 31, 2018, future minimum rental payments required under operating leases that have initial or remaining non-cancelable lease terms in excess of one year are as follows (in millions): Gross rental commitments Rentals from subleases Net rental commitments 2019 $ 303 $ (34 ) $ 269 2020 253 (30 ) 223 2021 221 (30 ) 191 2022 182 (30 ) 152 2023 148 (12 ) 136 Thereafter 472 (5 ) 467 Total minimum payments required $ 1,579 $ (141 ) $ 1,438
Lessor, Operating Lease, Payments to be Received, Maturity	Maturity analysis of operating and finance leases as of December 31, 2019 are as follows (in millions): Operating Leases Finance Leases Total 2020 $ 208 $ 26 $ 234 2021 202 26 228 2022 177 20 197 2023 141 18 159 2024 113 15 128 Thereafter 432 — 432 Total undiscounted future minimum lease payments 1,273 105 1,378 Less: Imputed interest (143 ) (5 ) (148 ) Present value of lease liabilities $ 1,130 $ 100 $ 1,230 At December 31, 2018, future minimum rental payments required under operating leases that have initial or remaining non-cancelable lease terms in excess of one year are as follows (in millions): Gross rental commitments Rentals from subleases Net rental commitments 2019 $ 303 $ (34 ) $ 269 2020 253 (30 ) 223 2021 221 (30 ) 191 2022 182 (30 ) 152 2023 148 (12 ) 136 Thereafter 472 (5 ) 467 Total minimum payments required $ 1,579 $ (141 ) $ 1,438

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income from continuing operations before income tax	Income before income tax from continuing operations and the provision for income tax from continuing operations consist of the following (in millions): Years ended December 31 2019 2018 2017 Income before income taxes: U.K. $ 228 $ (240 ) $ (420 ) U.S. (219 ) (601 ) (765 ) Other 1,862 2,087 1,870 Total $ 1,871 $ 1,246 $ 685 Income tax expense (benefit): Current: U.K. $ 20 $ 21 $ 1 U.S. federal 22 101 48 U.S. state and local 41 35 18 Other 250 214 201 Total current tax expense $ 333 $ 371 $ 268 Deferred tax expense (benefit): U.K. $ 35 $ 19 $ (5 ) U.S. federal (20 ) (165 ) 12 U.S. state and local (27 ) (56 ) (35 ) Other (24 ) (23 ) 10 Total deferred tax benefit $ (36 ) $ (225 ) $ (18 ) Total income tax expense $ 297 $ 146 $ 250
Reconciliation of the income tax provisions based on the U.S. statutory corporate tax rate to the provisions reflected in the Consolidated Financial Statements	The reconciliation to the provisions from continuing operations reflected in the Consolidated Financial Statements is as follows: Years ended December 31 2019 2018 2017 Statutory tax rate 19.0% 19.0% 19.3% U.S. state income taxes, net of U.S. federal benefit 0.5 (0.4) (1.5) Taxes on international operations (1) (6.0) (7.3) (30.3) Nondeductible expenses 1.6 2.7 3.4 Adjustments to prior year tax requirements 0.1 0.9 2.0 Adjustments to valuation allowances 1.8 3.8 (1.8) Change in uncertain tax positions 2.2 0.9 1.6 Excess tax benefits related to shared based compensation (2) (2.8) (3.6) (8.0) U.S. Tax Reform impact (3) (0.3) 7.1 51.2 Loss on disposition — (10.2) — Other — net (0.2) (1.2) 0.6 Effective tax rate 15.9% 11.7% 36.5% (1) The Company determines the adjustment for taxes on international operations based on the difference between the statutory tax rate applicable to earnings in each foreign jurisdiction and the enacted rate of 19.0% , 19.0% and 19.3% at December 31, 2019 , 2018 , and 2017 , respectively. The benefit to the Company’s effective income tax rate from taxes on international operations relates to benefits from lower-taxed global operations, primarily due to the use of global funding structures and the tax holiday in Singapore. The impact decreased from 2017 to 2018 primarily as a result of the decrease in the U.S. federal tax rate. (2) With the adoption of ASU 2016-09 in 2017, excess tax benefits and deficiencies from share-based payment transactions are recognized as income tax expense or benefit in the Company’s Consolidated Statements of Income. (3) The impact of the Tax Reform Act including the Transition Tax, the re-measurement of U.S. deferred tax assets and liabilities from 35% to 21%, withholding tax accruals, and the allocation of tax benefit between continuing operations and discontinued operations related to utilization of foreign tax credits.
Components of Aon's deferred tax assets and liabilities	The components of the Company’s deferred tax assets and liabilities are as follows (in millions): As of December 31 2019 2018 Deferred tax assets: Net operating loss, capital loss, interest, and tax credit carryforwards $ 440 $ 563 Employee benefit plans 373 351 Lease liabilities 247 — Accrued interest 116 — Other accrued expenses 68 98 Investment basis differences 28 28 Deferred revenue 24 29 Lease and service guarantees 3 5 Other 57 46 Total 1,356 1,120 Valuation allowance on deferred tax assets (200 ) (171 ) Total $ 1,156 $ 949 Deferred tax liabilities: Intangibles and property, plant and equipment $ (251 ) $ (310 ) Lease right-of-use asset (219 ) — Deferred costs (128 ) (143 ) Unremitted earnings (28 ) (30 ) Unrealized foreign exchange gains (26 ) (26 ) Other accrued expenses (25 ) (36 ) Other (33 ) (24 ) Total $ (710 ) $ (569 ) Net deferred tax asset $ 446 $ 380
Deferred income taxes (assets and liabilities netted by jurisdiction) as classified in the Consolidated Statements of Financial Position	Deferred income taxes (assets and liabilities have been netted by jurisdiction) have been classified in the Consolidated Statements of Financial Position as follows (in millions): As of December 31 2019 2018 Deferred tax assets — non-current $ 645 $ 561 Deferred tax liabilities — non-current (199 ) (181 ) Net deferred tax asset $ 446 $ 380
Summary of operating and capital loss carryforwards	The Company had the following net operating loss, capital loss, and interest carryforwards (in millions): As of December 31 2019 2018 U.K. Operating loss carryforwards $ 438 $ 541 Capital loss carryforwards $ 411 $ 400 Interest carryforwards $ 203 $ 53 U.S. Federal operating loss carryforwards $ 1 $ 2 Federal capital loss carryforwards $ 112 $ 367 Federal interest carryforwards $ 355 $ 424 State operating loss carryforwards $ 376 $ 315 State capital loss carryforwards $ 123 $ 221 State interest carryforwards $ 172 $ 227 Other Non-U.S. Operating loss carryforwards $ 308 $ 369 Capital loss carryforwards $ 29 $ 30 Interest carryforwards $ 159 $ 186
Reconciliation of the beginning and ending amount of unrecognized tax benefits	The following is a reconciliation of the Company’s beginning and ending amount of uncertain tax positions (in millions): 2019 2018 Balance at January 1 $ 279 $ 280 Additions based on tax positions related to the current year 23 18 Additions for tax positions of prior years 12 10 Reductions for tax positions of prior years (5 ) (24 ) Settlements (5 ) — Business combinations — 1 Lapse of statute of limitations (5 ) (6 ) Foreign currency translation — — Balance at December 31 $ 299 $ 279

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Schedule of Repurchase Agreements	The following table summarizes the Company’s share repurchase activity (in millions, except per share data): Twelve months ended December 31 2019 2018 Shares repurchased 10.5 10.0 Average price per share $ 186.33 $ 143.94 Costs recorded to retained earnings Total repurchase cost $ 1,950 $ 1,447 Additional associated costs 10 7 Total costs recorded to retained earnings $ 1,960 $ 1,454
Schedule of components of weighted average number of shares outstanding	Weighted average ordinary shares outstanding are as follows (in millions): Years ended December 31 2019 2018 2017 Basic weighted average ordinary shares outstanding 238.6 245.2 258.5 Dilutive effect of potentially issuable shares 2.0 1.8 2.2 Diluted weighted average ordinary shares outstanding 240.6 247.0 260.7
Components of Accumulated other comprehensive loss, net of related tax	Changes in Accumulated other comprehensive loss by component, net of related tax, are as follows (in millions): Change in Fair Value of Financial Instruments (1) Foreign Currency Translation Adjustments Postretirement Benefit Obligation (2) Total Balance at January 1, 2017 $ (37 ) $ (1,264 ) $ (2,611 ) $ (3,912 ) Other comprehensive income (loss) before reclassifications Other comprehensive income (loss) before reclassifications 18 397 (220 ) 195 Tax benefit (expense) (3 ) (5 ) 55 47 Other comprehensive income (loss) before reclassifications, net 15 392 (165 ) 242 Amounts reclassified from accumulated other comprehensive income (loss) Amounts reclassified from accumulated other comprehensive income (loss) (2 ) (7 ) 236 227 Tax benefit (expense) (1 ) — (52 ) (53 ) Amounts reclassified from accumulated other comprehensive income (loss), net (3 ) (7 ) 184 174 Net current period other comprehensive income (loss) 12 385 19 416 Balance at December 31, 2017 (25 ) (879 ) (2,592 ) (3,496 ) Adoption of new accounting guidance (3) (1 ) — — (1 ) Balance at January 1, 2018 (26 ) (879 ) (2,592 ) (3,497 ) Other comprehensive income (loss) before reclassifications Other comprehensive income (loss) before reclassifications (15 ) (437 ) (124 ) (576 ) Tax benefit (expense) 18 (3 ) 28 43 Other comprehensive income (loss) before reclassifications, net 3 (440 ) (96 ) (533 ) Amounts reclassified from accumulated other comprehensive income (loss) Amounts reclassified from accumulated other comprehensive income (loss) 11 — 146 157 Tax benefit (expense) (3 ) — (33 ) (36 ) Amounts reclassified from accumulated other comprehensive income (loss), net 8 — 113 121 Net current period other comprehensive income (loss) 11 (440 ) 17 (412 ) Balance at December 31, 2018 (15 ) (1,319 ) (2,575 ) (3,909 ) Other comprehensive income (loss) before reclassifications Other comprehensive income (loss) before reclassifications (10 ) 15 (287 ) (282 ) Tax benefit (expense) 1 (1 ) 58 58 Other comprehensive income (loss) before reclassifications, net (9 ) 14 (229 ) (224 ) Amounts reclassified from accumulated other comprehensive income (loss) Amounts reclassified from accumulated other comprehensive income (loss) 14 — 109 123 Tax benefit (expense) (2 ) — (21 ) (23 ) Amounts reclassified from accumulated other comprehensive income (loss), net 12 — 88 100 Net current period other comprehensive income (loss) 3 14 (141 ) (124 ) Balance at December 31, 2019 $ (12 ) $ (1,305 ) $ (2,716 ) $ (4,033 ) (1) Reclassifications from this category included in Accumulated other comprehensive loss are recorded in Revenue, Interest expense, and Compensation and benefits in the accompanying Consolidated Statements of Income. See Note 15 “Derivatives and Hedging” for additional information regarding the Company’s derivative and hedging activity. (2) Reclassifications from this category included in Accumulated other comprehensive loss are recorded in Other income (expense). (3) Refer to Note 2 “Summary of Significant Accounting Principles and Practices ” for further information.

Employee Benefits (Tables)

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Expense recognized for defined contribution savings plans, included in compensation and benefits and discontinued operations in the consolidated statements of income	The expense for the significant plans in the U.S., U.K., Netherlands and Canada is as follows (in millions): Years ended December 31 2019 2018 2017 U.S. $ 98 $ 98 $ 105 U.K. 41 45 43 Netherlands and Canada 25 25 25 Total $ 164 $ 168 $ 173
Changes in Projected Benefit Obligations Fair Value of Plan Assets, Funded Status and Amount Recognized	The following tables provide a reconciliation of the changes in the projected benefit obligations and fair value of assets for the years ended December 31, 2019 and 2018 , and a statement of the funded status as of December 31, 2019 and 2018 , for Aon’s significant U.K., U.S., and other major pension plans, which are located in the Netherlands and Canada. These plans represent approximately 91% of the Company’s projected benefit obligations. U.K. U.S. Other (millions) 2019 2018 2019 2018 2019 2018 Change in projected benefit obligation At January 1 $ 4,129 $ 4,893 $ 2,877 $ 3,155 $ 1,271 $ 1,401 Service cost — — — — — — Interest cost 109 109 108 99 27 27 Plan amendment 10 13 — — — — Settlements (22 ) (176 ) — — — — Actuarial loss (gain) 594 (297 ) 373 (221 ) 177 (47 ) Benefit payments (168 ) (160 ) (166 ) (156 ) (42 ) (43 ) Foreign currency impact 127 (253 ) — — (8 ) (67 ) As of December 31 $ 4,779 $ 4,129 $ 3,192 $ 2,877 $ 1,425 $ 1,271 Accumulated benefit obligation at end of year $ 4,779 $ 4,129 $ 3,192 $ 2,877 $ 1,391 $ 1,247 Change in fair value of plan assets At January 1 $ 5,225 $ 5,906 $ 1,796 $ 1,958 $ 1,155 $ 1,256 Actual return on plan assets 687 (125 ) 398 (141 ) 182 (19 ) Employer contributions 78 97 38 135 19 20 Settlements (22 ) (176 ) — — — — Benefit payments (168 ) (160 ) (166 ) (156 ) (42 ) (43 ) Foreign currency impact 159 (317 ) — — (11 ) (59 ) As of December 31 $ 5,959 $ 5,225 $ 2,066 $ 1,796 $ 1,303 $ 1,155 Market related value at end of year $ 5,959 $ 5,225 $ 1,969 $ 1,981 $ 1,303 $ 1,155 Amount recognized in Statement of Financial Position as of December 31 Funded status $ 1,180 $ 1,096 $ (1,126 ) $ (1,081 ) $ (122 ) $ (116 ) Unrecognized prior-service cost 40 30 1 3 (6 ) (7 ) Unrecognized loss 1,204 1,106 1,762 1,705 460 440 Net amount recognized $ 2,424 $ 2,232 $ 637 $ 627 $ 332 $ 317 The following table provides an overview of the accumulated projected benefit obligation, fair value of plan assets, funded status and net amount recognized as of December 31, 2019 and 2018 for the Company’s other significant postretirement benefit plans located in the U.S. and Canada (in millions): 2019 2018 Accumulated projected benefit obligation $ 103 $ 91 Fair value of plan assets 16 14 Funded status (87 ) (77 ) Unrecognized prior-service credit (1 ) (1 ) Unrecognized (gain) loss 3 (6 ) Net amount recognized $ (85 ) $ (84 )
Amounts recognized in the Consolidated Statements of Financial Position	Amounts recognized in the Consolidated Statements of Financial Position consist of (in millions): U.K. U.S. Other 2019 2018 2019 2018 2019 2018 Prepaid benefit cost (1) $ 1,200 $ 1,113 $ — $ — $ — $ — Accrued benefit liability - current (2) (1 ) (1 ) (50 ) (46 ) (5 ) (5 ) Accrued benefit liability - non-current (3) (19 ) (16 ) (1,076 ) (1,035 ) (117 ) (111 ) Accumulated other comprehensive loss 1,244 1,136 1,763 1,708 454 433 Net amount recognized $ 2,424 $ 2,232 $ 637 $ 627 $ 332 $ 317 (1) Included in Prepaid pension (2) Included in Other current liabilities (3) Included in Pension, other postretirement, and postemployment liabilities
Amounts recognized in Accumulated other comprehensive loss that have not yet been recognized as components of net periodic benefit cost	Amounts recognized in Accumulated other comprehensive loss (income) that have not yet been recognized as components of net periodic benefit cost at December 31, 2019 and 2018 consist of (in millions): U.K. U.S. Other 2019 2018 2019 2018 2019 2018 Net loss $ 1,204 $ 1,106 $ 1,762 $ 1,705 $ 460 $ 440 Prior service cost (income) 40 30 1 3 (6 ) (7 ) Total $ 1,244 $ 1,136 $ 1,763 $ 1,708 $ 454 $ 433
Components of net periodic benefit cost for the pension plans	Service cost is reported in Compensation and benefits and all other components are reported in Other income (expense) as follows (in millions): U.K. U.S. Other 2019 2018 2017 2019 2018 2017 2019 2018 2017 Service cost $ — $ — $ — $ — $ — $ — $ — $ — $ — Interest cost 109 109 123 108 99 96 27 27 26 Expected return on plan assets, net of administration expenses (191 ) (192 ) (199 ) (136 ) (144 ) (140 ) (40 ) (45 ) (47 ) Amortization of prior-service cost 1 1 1 2 2 2 — — — Amortization of net actuarial loss 29 28 31 53 59 50 12 12 11 Net periodic benefit (income) cost (52 ) (54 ) (44 ) 27 16 8 (1 ) (6 ) (10 ) Settlement expense 5 37 125 — — — — — — Total net periodic benefit cost (income) $ (47 ) $ (17 ) $ 81 $ 27 $ 16 $ 8 $ (1 ) $ (6 ) $ (10 )
Weighted-average assumptions used to determine future benefit obligations and net periodic benefit cost	The weighted-average assumptions used to determine benefit obligations are as follows: U.K. U.S. (1) Other 2019 2018 2019 2018 2019 2018 Discount rate 2.09% 2.95% 2.72 - 3.17% 3.92 - 4.26% 0.91 - 3.10% 1.89 - 3.88% Rate of compensation increase 3.24 - 3.74% 3.73 - 4.23% N/A N/A 1.00 - 3.00% 1.00 - 3.00% Underlying price inflation 1.78% 1.88% N/A N/A 2.00% 2.00% (1) U.S. pension plans are frozen and therefore not impacted by compensation increases or price inflation. The weighted-average assumptions used to determine the net periodic benefit cost are as follows: U.K. U.S. Other 2019 2018 2017 2019 2018 2017 2019 2018 2017 Discount rate 2.95% 2.63% 2.77% 3.92 - 4.26% 3.27 - 3.61% 3.53 - 4.11% 1.89 - 3.88% 1.78 - 3.39% 1.85 - 3.81% Expected return on plan assets, net of administration expenses 3.64% 3.34% 3.36% 7.05% 7.71% 7.88% 2.50 - 4.10% 1.70 - 4.85% 2.68 - 5.15% Rate of compensation increase 3.73 - 4.23% 3.70 - 4.20% 3.70 - 4.20% N/A N/A N/A 1.00 - 3.00% 1.00 - 3.00% 1.00 - 3.50%
Fair values of pension plan assets	The fair values of the Company’s other major pension plan assets at December 31, 2019 and December 31, 2018 , by asset category, are as follows (in millions): Fair Value Measurements Using Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 5 $ 5 $ — $ — Equity investments: Pooled funds (2) 323 — — — Fixed income investments: Pooled funds (2) 907 — — — Other investments: Alternative investments (2) (3) 62 — — — Real estate (2) (4) 6 — — — Total $ 1,303 $ 5 $ — $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of limited partnerships, private equity, and hedge funds. (4) Consists of property funds and trusts holding direct real estate investments. Fair Value Measurements Using Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 10 $ 10 $ — $ — Equity investments: Pooled funds (2) 281 — — — Fixed income investments: Derivatives 9 — 9 — Pooled funds (2) 782 — — — Other investments: Alternative investments (2) (3) 63 — — — Real estate (2) (4) 10 — — — Total $ 1,155 $ 10 $ 9 $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of limited partnerships, private equity, and hedge funds. (4) Consists of property funds and trusts holding direct real estate investments. The fair values of the Company’s major U.K. pension plan assets at December 31, 2019 and December 31, 2018 , by asset category, are as follows (in millions): Fair Value Measurements Using Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 81 $ 81 $ — $ — Equity investments: Pooled funds (2) 119 — — — Fixed income investments: Derivatives (3) (1,205 ) — (1,205 ) — Government and agency bonds 2,667 2,667 — — Annuities 1,849 — — 1,849 Pooled funds (2) 1,486 — — — Other investments: Real estate (2) (4) 180 — — — Alternative investments (2) (5) 782 — — — Total $ 5,959 $ 2,748 $ (1,205 ) $ 1,849 (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of equity securities and equity derivatives, including repurchase agreements. (4) Consists of property funds and trusts holding direct real estate investments. (5) Consists of limited partnerships, private equity, and hedge funds. Fair Value Measurements Using Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 96 $ 96 $ — $ — Equity investments: Pooled funds (2) 212 — — — Fixed income investments: Derivatives (3) (949 ) — (949 ) — Corporate bonds 367 — 367 — Government and agency bonds 2,079 2,079 — — Annuities 1,688 — — 1,688 Pooled funds (2) 889 — — — Other investments: Real estate (2) (4) 149 — — — Alternative investments (2) (5) 694 — — — Total $ 5,225 $ 2,175 $ (582 ) $ 1,688 (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of equity securities and equity derivatives, including repurchase agreements. (4) Consists of property funds and trusts holding direct real estate investments. (5) Consists of limited partnerships, private equity, and hedge funds. The fair values of the Company’s U.S. pension plan assets at December 31, 2019 and December 31, 2018 , by asset category, are as follows (in millions): Fair Value Measurements Using Asset Category Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 77 $ 77 $ — $ — Equity investments: Equity securities 195 195 — — Equity derivatives 22 — 22 — Pooled funds (2) 583 — — — Fixed income investments: Corporate bonds 128 — 128 — Government and agency bonds 199 162 37 — Asset-backed securities 3 — 3 — Pooled funds (2) 545 — — — Other investments: Real estate (2) (3) 133 — — — Alternative investments (2) (4) 181 — — — Total $ 2,066 $ 434 $ 190 $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of property funds and trusts holding direct real estate investments. (4) Consists of limited partnerships, private equity, and hedge funds. Fair Value Measurements Using Asset Category Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Cash and cash equivalents (1) $ 130 $ 130 $ — $ — Equity investments: Equity securities 384 384 — — Equity derivatives (14 ) — (14 ) — Pooled funds (2) 285 — — — Fixed income investments: Corporate bonds 111 — 111 — Government and agency bonds 126 95 31 — Asset-backed securities 2 — 2 — Pooled funds (2) 417 — — — Other investments: Real estate and REITs (3) 78 78 — — Alternative investments (2) (4) 277 — — — Total $ 1,796 $ 687 $ 130 $ — (1) Consists of cash and institutional short-term investment funds. (2) Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note. (3) Consists of exchange traded real estate investment trusts (“REITs”). (4) Consists of limited partnerships, private equity, and hedge funds.
Changes in the Level 3 fair-value category	The following table presents the changes in the Level 3 fair-value category in the Company’s U.K. pension plans for the years ended December 31, 2019 and December 31, 2018 (in millions): Fair Value Measurements Using Level 3 Inputs Annuities Balance at January 1, 2018 $ 1,909 Actual return on plan assets: Relating to assets still held at December 31, 2018 (122 ) Purchases, sales and settlements—net 7 Foreign exchange (106 ) Balance at December 31, 2018 1,688 Actual return on plan assets: Relating to assets still held at December 31, 2019 113 Foreign exchange 48 Balance at December 31, 2019 $ 1,849
Estimated Future Benefit Payments	Estimated future benefit payments for plans, not including voluntary one-time lump sum payments, are as follows at December 31, 2019 (in millions): U.K. U.S. Other 2020 $ 164 $ 186 $ 43 2021 $ 154 $ 190 $ 44 2022 $ 160 $ 192 $ 46 2023 $ 167 $ 189 $ 46 2024 $ 172 $ 181 $ 47 2025 – 2029 $ 913 $ 887 $ 252
Other information related to the Company's other post-retirement benefit plans	Other information related to the Company’s other postretirement benefit plans are as follows: 2019 2018 2017 Net periodic benefit cost recognized (millions) $3 $3 $1 Weighted-average discount rate used to determine future benefit obligations 2.93 - 3.25% 3.91 - 4.26% 3.32 - 3.64% Weighted-average discount rate used to determine net periodic benefit costs 3.91 - 4.26% 3.32 - 3.64% 3.71 - 4.15%

Share-based Compensation Plans

Share-based Compensation Plans (Tables)	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Share-based compensation expense recognized in continuing operations	The following table summarizes share-based compensation expense recognized in the Consolidated Statements of Income in Compensation and benefits (in millions): Years ended December 31 2019 2018 2017 Restricted share units (“RSUs”) $ 198 $ 186 $ 182 Performance share awards (“PSAs”) 110 143 127 Employee share purchase plans 9 9 10 Total share-based compensation expense 317 338 319 Tax benefit 66 74 73 Share-based compensation expense, net of tax $ 251 $ 264 $ 246
Restricted share unit activity	The following table summarizes the status of the Company’s RSUs, including shares related to the Divested Business (shares in thousands, except fair value): 2019 2018 2017 Years ended December 31 Shares Fair Value at Date of Grant (1) Shares Fair Value at Date of Grant (1) Shares Fair Value at Date of Grant (1) Non-vested at beginning of year 4,208 $ 120 4,849 $ 104 6,195 $ 89 Granted 1,306 $ 175 1,500 $ 141 1,700 $ 123 Vested (1,661 ) $ 113 (1,943 ) $ 97 (2,407 ) $ 82 Forfeited (219 ) $ 131 (198 ) $ 114 (639 ) $ 93 Non-vested at end of year 3,634 $ 143 4,208 $ 120 4,849 $ 104 (1) Represents per share weighted average fair value of award at date of grant.
Schedule of performance-based plans information	The following table summarizes the Company’s target PSAs granted and shares that would be issued at current performance levels for PSAs granted during the years ended December 31, 2019 , 2018 , and 2017 , respectively (shares in thousands and dollars in millions, except fair value): 2019 2018 2017 Target PSAs granted during period 467 564 548 Weighted average fair value per share at date of grant $ 165 $ 134 $ 114 Number of shares that would be issued based on current performance levels 453 818 1,057 Unamortized expense, based on current performance levels $ 50 $ 34 $ —

Derivatives and Hedging (Tables

Derivatives and Hedging (Tables)	12 Months Ended
	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Notional and fair values of derivative instruments	The notional and fair values of derivative instruments are as follows (in millions): Notional Amount Net Amount of Derivative Assets Presented in the Statements of Financial Position (1) Net Amount of Derivative Liabilities Presented in the Statements of Financial Position (2) As of December 31 2019 2018 2019 2018 2019 2018 Foreign exchange contracts Accounted for as hedges $ 579 $ 646 $ 16 $ 17 $ 1 $ 2 Not accounted for as hedges (3) 297 269 2 1 — 6 Total $ 876 $ 915 $ 18 $ 18 $ 1 $ 8 (1) Included within Other current assets ( $7 million in 2019 and $3 million in 2018 ) or Other non-current assets ( $11 million in 2019 and $15 million in 2018 ). (2) Included within Other current liabilities ( $1 million in 2019 and $5 million in 2018 ) or Other non-current liabilities ( $3 million in 2018 ). (3) These contracts typically are for 30 -day durations and executed close to the last day of the most recent reporting month, thereby resulting in nominal fair values at the balance sheet date.
Derivative gains (losses)	The amounts of derivative gains (losses) recognized in the Consolidated Financial Statements are as follows (in millions): 2019 2018 2017 (Loss) Gain recognized in Accumulated other comprehensive loss $ (9 ) $ (18 ) $ 18 The amounts of derivative gains (losses) reclassified from Accumulated other comprehensive loss into Consolidated Statements of Income are as follows (in millions): Years Ended December 31 2019 2018 2017 Total revenue (1) $ (12 ) $ — $ — Compensation and benefits (1 ) 1 14 Other general expense — (2 ) (5 ) Interest expense (1 ) (2 ) (1 ) Other income (expense) (1) — (8 ) (9 ) Total $ (14 ) $ (11 ) $ (1 ) (1) With the adoption of new hedge accounting guidance in the first quarter of 2019, gains (losses) on derivatives accounted for as hedges are recognized in Total revenue in the Company’s Consolidated Statement of Income rather than Other income (expense). Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for additional details.

Fair Value Measurements and F_2

Fair Value Measurements and Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of assets and liabilities that are measured at fair value on a recurring basis	The following tables present the categorization of the Company’s assets and liabilities that are measured at fair value on a recurring basis at December 31, 2019 and December 31, 2018 (in millions): Fair Value Measurements Using Balance at December 31, 2019 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Money market funds (1) $ 2,007 $ 2,007 $ — $ — Other investments Government bonds $ 1 $ — $ 1 $ — Equity investments $ 1 $ — $ 1 $ — Derivatives (2) Gross foreign exchange contracts $ 21 $ — $ 21 $ — Liabilities Derivatives (2) Gross foreign exchange contracts $ 4 $ — $ 4 $ — Fair Value Measurements Using Balance at December 31, 2018 Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Assets Money market funds (1) $ 1,759 $ 1,759 $ — $ — Other investments Government bonds $ 1 $ — $ 1 $ — Equity investments $ 2 $ — $ 2 $ — Derivatives (2) Gross foreign exchange contracts $ 21 $ — $ 21 $ — Liabilities Derivatives (2) Gross foreign exchange contracts $ 12 $ — $ 12 $ — (1) Included within Fiduciary assets or Short-term investments in the Consolidated Statements of Financial Position, depending on their nature and initial maturity. (2) Refer to Note 15 “Derivatives and Hedging” for additional information regarding the Company’s derivatives and hedging activity.
Schedule of financial instruments where the carrying amounts and fair values differ	The fair value of debt is classified as Level 2 of the fair value hierarchy. The following table provides the carrying value and fair value for the Company’s term debt (in millions): 2019 2018 As of December 31 Carrying Value Fair Value Carrying Value Fair Value Current portion of long-term debt $ 600 $ 614 $ — $ — Long-term debt $ 6,627 $ 7,442 $ 5,993 $ 6,159

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2019
Segment Reporting [Abstract]
Schedule of Disclosure on Geographic Areas of Fixed Assets and Operating Lease Right-of-Use Asset	Consolidated long-lived assets, net by geographic area are as follows (in millions): Years ended December 31 Total United States Americas other than U.S. United Kingdom Europe, Middle East, & Africa Asia Pacific 2019 (1) $ 1,550 $ 686 $ 145 $ 225 $ 319 $ 175 2018 $ 588 $ 288 $ 44 $ 58 $ 101 $ 97 (1) In the first quarter of 2019, Aon adopted new accounting guidance related to the treatment of leases which was applied under the modified retrospective approach. Refer to Note 2 “Summary of Significant Accounting Principles and Practices” for further information.

Guarantee of Registered Secur_2

Guarantee of Registered Securities (Tables)	12 Months Ended
	Dec. 31, 2019
Guarantee of Registered Securities
Condensed Consolidating Statement of Income	Condensed Consolidating Statement of Income Year Ended December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Revenue Total revenue $ — $ — $ 11,013 $ — $ 11,013 Expenses Compensation and benefits 49 20 5,985 — 6,054 Information technology — — 494 — 494 Premises — 23 316 — 339 Depreciation of fixed assets — — 172 — 172 Amortization and impairment of intangible assets — — 392 — 392 Other general expense 10 4 1,379 — 1,393 Total operating expenses 59 47 8,738 — 8,844 Operating income (loss) (59 ) (47 ) 2,275 — 2,169 Interest income — 39 — (31 ) 8 Interest expense (188 ) (133 ) (17 ) 31 (307 ) Intercompany interest income (expense) 21 (464 ) 443 — — Intercompany other income (expense) 185 (362 ) 177 — — Other income (expense) 18 (60 ) 51 (8 ) 1 Income (loss) from continuing operations before income taxes (23 ) (1,027 ) 2,929 (8 ) 1,871 Income tax expense (benefit) (12 ) (182 ) 491 — 297 Net income (loss) from continuing operations (11 ) (845 ) 2,438 (8 ) 1,574 Net income (loss) from discontinued operations — — (1 ) — (1 ) Net income (loss) before equity in earnings of subsidiaries (11 ) (845 ) 2,437 (8 ) 1,573 Equity in earnings of subsidiaries 1,551 1,254 409 (3,214 ) — Net income 1,540 409 2,846 (3,222 ) 1,573 Less: Net income attributable to noncontrolling interests — — 41 — 41 Net income attributable to Aon shareholders $ 1,540 $ 409 $ 2,805 $ (3,222 ) $ 1,532 Condensed Consolidating Statement of Income Year Ended December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Revenue Total revenue $ — $ — $ 10,770 $ — $ 10,770 Expenses Compensation and benefits 74 (1 ) 6,030 — 6,103 Information technology — — 484 — 484 Premises — — 370 — 370 Depreciation of fixed assets — — 176 — 176 Amortization and impairment of intangible assets — — 593 — 593 Other general expense 4 90 1,406 — 1,500 Total operating expenses 78 89 9,059 — 9,226 Operating income (loss) (78 ) (89 ) 1,711 — 1,544 Interest income — 58 — (53 ) 5 Interest expense (203 ) (101 ) (27 ) 53 (278 ) Intercompany interest income (expense) 15 (514 ) 499 — — Intercompany other income (expense) 65 (373 ) 308 — — Other income (expense) 41 (48 ) 3 (21 ) (25 ) Income (loss) from continuing operations before income taxes (160 ) (1,067 ) 2,494 (21 ) 1,246 Income tax expense (benefit) (60 ) (192 ) 398 — 146 Net income (loss) from continuing operations (100 ) (875 ) 2,096 (21 ) 1,100 Net income (loss) from discontinued operations — — 74 — 74 Net income (loss) before equity in earnings of subsidiaries (100 ) (875 ) 2,170 (21 ) 1,174 Equity in earnings of subsidiaries 1,255 1,009 134 (2,398 ) — Net income 1,155 134 2,304 (2,419 ) 1,174 Less: Net income attributable to noncontrolling interests — — 40 — 40 Net income attributable to Aon shareholders $ 1,155 $ 134 $ 2,264 $ (2,419 ) $ 1,134 Condensed Consolidating Statement of Income Year Ended December 31, 2017 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Revenue Total revenue $ — $ — $ 9,998 $ — $ 9,998 Expenses Compensation and benefits 150 26 5,827 — 6,003 Information technology — — 419 — 419 Premises — — 348 — 348 Depreciation of fixed assets — — 187 — 187 Amortization and impairment of intangible assets — — 704 — 704 Other general expense 12 20 1,240 — 1,272 Total operating expenses 162 46 8,725 — 8,933 Operating income (loss) (162 ) (46 ) 1,273 — 1,065 Interest income — 52 4 (29 ) 27 Interest expense (202 ) (94 ) (15 ) 29 (282 ) Intercompany interest income (expense) 14 (543 ) 529 — — Intercompany other income (expense) 282 (385 ) 103 — — Other income (expense) (62 ) 12 (93 ) 18 (125 ) Income (loss) from continuing operations before income taxes (130 ) (1,004 ) 1,801 18 685 Income tax expense (benefit) (43 ) (110 ) 403 — 250 Net income (loss) from continuing operations (87 ) (894 ) 1,398 18 435 Net income (loss) from discontinued operations — — 828 — 828 Net income (loss) before equity in earnings of subsidiaries (87 ) (894 ) 2,226 18 1,263 Equity in earnings of subsidiaries 1,295 1,141 247 (2,683 ) — Net income 1,208 247 2,473 (2,665 ) 1,263 Less: Net income attributable to noncontrolling interests — — 37 — 37 Net income attributable to Aon shareholders $ 1,208 $ 247 $ 2,436 $ (2,665 ) $ 1,226
Condensed Consolidating Statement of Comprehensive Income	Condensed Consolidating Statement of Comprehensive Income Year Ended December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Net income $ 1,540 $ 409 $ 2,846 $ (3,222 ) $ 1,573 Less: Net income attributable to noncontrolling interests — — 41 — 41 Net income attributable to Aon shareholders 1,540 409 2,805 (3,222 ) 1,532 Other comprehensive income (loss), net of tax: Change in fair value of financial instruments — 3 — — 3 Foreign currency translation adjustments — — 6 8 14 Postretirement benefit obligation — (44 ) (97 ) — (141 ) Total other comprehensive income (loss) — (41 ) (91 ) 8 (124 ) Equity in other comprehensive income (loss) of subsidiaries, net of tax (132 ) (131 ) (172 ) 435 — Less: Other comprehensive income (loss) attributable to noncontrolling interests — — — — — Total other comprehensive income (loss) attributable to Aon shareholders (132 ) (172 ) (263 ) 443 (124 ) Comprehensive income (loss) attributable to Aon shareholders $ 1,408 $ 237 $ 2,542 $ (2,779 ) $ 1,408 Condensed Consolidating Statement of Comprehensive Income Year Ended December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Net income $ 1,155 $ 134 $ 2,304 $ (2,419 ) $ 1,174 Less: Net income attributable to noncontrolling interests — — 40 — 40 Net income attributable to Aon shareholders 1,155 134 2,264 (2,419 ) 1,134 Other comprehensive income (loss), net of tax: Change in fair value of financial instruments — — 11 — 11 Foreign currency translation adjustments — — (465 ) 21 (444 ) Postretirement benefit obligation — (2 ) 19 — 17 Total other comprehensive income (loss) — (2 ) (435 ) 21 (416 ) Equity in other comprehensive income (loss) of subsidiaries, net of tax (433 ) (415 ) (417 ) 1,265 — Less: Other comprehensive income (loss) attributable to noncontrolling interests — — (4 ) — (4 ) Total other comprehensive income (loss) attributable to Aon shareholders (433 ) (417 ) (848 ) 1,286 (412 ) Comprehensive income (loss) attributable to Aon shareholders $ 722 $ (283 ) $ 1,416 $ (1,133 ) $ 722 Condensed Consolidating Statement of Comprehensive Income Year Ended December 31, 2017 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Net income $ 1,208 $ 247 $ 2,473 $ (2,665 ) $ 1,263 Less: Net income attributable to noncontrolling interests — — 37 — 37 Net income attributable to Aon shareholders 1,208 247 2,436 (2,665 ) 1,226 Other comprehensive income (loss), net of tax: Change in fair value of financial instruments — 3 9 — 12 Foreign currency translation adjustments — — 408 (18 ) 390 Postretirement benefit obligation — (101 ) 120 — 19 Total other comprehensive income (loss) — (98 ) 537 (18 ) 421 Equity in other comprehensive income (loss) of subsidiaries, net of tax 434 515 417 (1,366 ) — Less: Other comprehensive income (loss) attributable to noncontrolling interests — — 5 — 5 Total other comprehensive income (loss) attributable to Aon shareholders 434 417 949 (1,384 ) 416 Comprehensive income (loss) attributable to Aon shareholders $ 1,642 $ 664 $ 3,385 $ (4,049 ) $ 1,642
Condensed Consolidating Statement of Financial Position	Condensed Consolidating Statement of Financial Position As of December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Assets Current assets Cash and cash equivalents $ — $ 2,271 $ 468 $ (1,949 ) $ 790 Short-term investments — 28 110 — 138 Receivables, net — — 3,112 — 3,112 Fiduciary assets — — 11,834 — 11,834 Intercompany receivables 246 1,214 12,799 (14,259 ) — Other current assets — 7 595 — 602 Total current assets 246 3,520 28,918 (16,208 ) 16,476 Goodwill — — 8,165 — 8,165 Intangible assets, net — — 783 — 783 Operating lease right-of-use assets — 110 819 — 929 Fixed assets, net — — 621 — 621 Deferred tax assets 89 577 169 (190 ) 645 Intercompany receivables 868 261 7,046 (8,175 ) — Prepaid pension — 7 1,209 — 1,216 Other non-current assets — 32 538 — 570 Investment in subsidiary 8,899 19,470 (957 ) (27,412 ) — Total assets $ 10,102 $ 23,977 $ 47,311 $ (51,985 ) $ 29,405 Liabilities and equity Liabilities Current liabilities Accounts payable and accrued liabilities $ 2,157 $ 56 $ 1,675 $ (1,949 ) $ 1,939 Short-term debt and current portion of long-term debt 112 600 — — 712 Fiduciary liabilities — — 11,834 — 11,834 Intercompany payables 234 12,978 1,047 (14,259 ) — Other current liabilities — 80 1,006 — 1,086 Total current liabilities 2,503 13,714 15,562 (16,208 ) 15,571 Long-term debt 4,223 2,404 — — 6,627 Non-current operating lease liabilities — 143 801 — 944 Deferred tax liabilities — — 389 (190 ) 199 Pension, other postretirement, and other post-employment liabilities — 1,348 390 — 1,738 Intercompany payables — 7,212 963 (8,175 ) — Other non-current liabilities 1 113 763 — 877 Total liabilities 6,727 24,934 18,868 (24,573 ) 25,956 Equity Total Aon shareholders' equity 3,375 (957 ) 28,369 (27,412 ) 3,375 Noncontrolling interests — — 74 — 74 Total equity 3,375 (957 ) 28,443 (27,412 ) 3,449 Total liabilities and equity $ 10,102 $ 23,977 $ 47,311 $ (51,985 ) $ 29,405 Condensed Consolidating Statement of Financial Position As of December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Assets Current assets Cash and cash equivalents $ — $ 862 $ 575 $ (781 ) $ 656 Short-term investments — 56 116 — 172 Receivables, net — — 2,760 — 2,760 Fiduciary assets — — 10,166 — 10,166 Intercompany receivables 191 897 11,634 (12,722 ) — Other current assets — 16 602 — 618 Total current assets 191 1,831 25,853 (13,503 ) 14,372 Goodwill — — 8,171 — 8,171 Intangible assets, net — — 1,149 — 1,149 Fixed assets, net — — 588 — 588 Deferred tax assets 94 467 144 (144 ) 561 Intercompany receivables 403 261 7,225 (7,889 ) — Prepaid pension — 5 1,128 — 1,133 Other non-current assets 1 30 417 — 448 Investment in subsidiary 8,433 19,031 (983 ) (26,481 ) — Total assets $ 9,122 $ 21,625 $ 43,692 $ (48,017 ) $ 26,422 Liabilities and equity Liabilities Current liabilities Accounts payable and accrued liabilities $ 274 $ 70 $ 2,380 $ (781 ) $ 1,943 Short-term debt and current portion of long-term debt 250 — 1 — 251 Fiduciary liabilities — — 10,166 — 10,166 Intercompany payables 213 11,875 634 (12,722 ) — Other current liabilities — 69 867 — 936 Total current liabilities 737 12,014 14,048 (13,503 ) 13,296 Long-term debt 4,231 1,762 — — 5,993 Deferred tax liabilities — — 325 (144 ) 181 Pension, other postretirement, and other post-employment liabilities — 1,275 361 — 1,636 Intercompany payables — 7,390 499 (7,889 ) — Other non-current liabilities 3 167 927 — 1,097 Total liabilities 4,971 22,608 16,160 (21,536 ) 22,203 Equity Total Aon shareholders' equity 4,151 (983 ) 27,464 (26,481 ) 4,151 Noncontrolling interests — — 68 — 68 Total equity 4,151 (983 ) 27,532 (26,481 ) 4,219 Total liabilities and equity $ 9,122 $ 21,625 $ 43,692 $ (48,017 ) $ 26,422
Condensed Consolidating Statement of Cash Flows	Condensed Consolidating Statement of Cash Flows Year Ended December 31, 2019 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Cash flows from operating activities Cash provided by (used for) operating activities - continuing operations $ 280 $ (140 ) $ 2,408 $ (713 ) $ 1,835 Cash provided by operating activities - discontinued operations — — — — — Cash provided by (used for) operating activities 280 (140 ) 2,408 (713 ) 1,835 Cash flows from investing activities Proceeds from investments — 29 189 (157 ) 61 Payments for investments — (62 ) (52 ) 1 (113 ) Net sales (purchases) of short-term investments — non-fiduciary — 28 7 — 35 Acquisition of businesses, net of cash acquired — — (39 ) — (39 ) Sale of businesses, net of cash sold — — 52 — 52 Capital expenditures — — (225 ) — (225 ) Cash used for investing activities - continuing operations — (5 ) (68 ) (156 ) (229 ) Cash used for investing activities - discontinued operations — — — — — Cash used for investing activities — (5 ) (68 ) (156 ) (229 ) Cash flows from financing activities Share repurchase (1,960 ) — — — (1,960 ) Advances from (to) affiliates 2,350 314 (2,365 ) (299 ) — Issuance of shares for employee benefit plans (131 ) — — — (131 ) Issuance of debt 2,739 3,313 — — 6,052 Repayment of debt (2,868 ) (2,073 ) — — (4,941 ) Cash dividends to shareholders (410 ) — — — (410 ) Noncontrolling interests and other financing activities — — (103 ) — (103 ) Cash provided by (used for) financing activities - continuing operations (280 ) 1,554 (2,468 ) (299 ) (1,493 ) Cash used for financing activities - discontinued operations — — — — — Cash provided by (used for) financing activities (280 ) 1,554 (2,468 ) (299 ) (1,493 ) Effect of exchange rate changes on cash and cash equivalents — — 21 — 21 Net increase (decrease) in cash and cash equivalents — 1,409 (107 ) (1,168 ) 134 Cash and cash equivalents at beginning of year — 862 575 (781 ) 656 Cash and cash equivalents at end of year $ — $ 2,271 $ 468 $ (1,949 ) $ 790 Condensed Consolidating Statement of Cash Flows Year Ended December 31, 2018 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Cash flows from operating activities Cash provided by operating activities - continuing operations $ 1,575 $ 3 $ 3,608 $ (3,500 ) $ 1,686 Cash provided by operating activities - discontinued operations — — — — — Cash provided by operating activities 1,575 3 3,608 (3,500 ) 1,686 Cash flows from investing activities Proceeds from investments — 24 955 (908 ) 71 Payments for investments (13 ) (47 ) (33 ) 13 (80 ) Net sales (purchases) of short-term investments — non-fiduciary — 299 49 — 348 Acquisition of businesses, net of cash acquired — — (58 ) — (58 ) Sale of businesses, net of cash sold — — (10 ) — (10 ) Capital expenditures — — (240 ) — (240 ) Cash provided by (used for) investing activities - continuing operations (13 ) 276 663 (895 ) 31 Cash used for investing activities - discontinued operations — — — — — Cash provided by (used for) investing activities (13 ) 276 663 (895 ) 31 Cash flows from financing activities Share repurchase (1,470 ) — — — (1,470 ) Advances from (to) affiliates 156 (2,291 ) (4,041 ) 6,176 — Issuance of shares for employee benefit plans (149 ) — — — (149 ) Issuance of debt 1,723 4,028 3 — 5,754 Repayment of debt (1,441 ) (3,678 ) (298 ) — (5,417 ) Cash dividends to shareholders (382 ) — — — (382 ) Noncontrolling interests and other financing activities — — (35 ) — (35 ) Cash used for financing activities - continuing operations (1,563 ) (1,941 ) (4,371 ) 6,176 (1,699 ) Cash used for financing activities - discontinued operations — — — — — Cash used for financing activities (1,563 ) (1,941 ) (4,371 ) 6,176 (1,699 ) Effect of exchange rate changes on cash and cash equivalents — — (118 ) — (118 ) Net decrease in cash and cash equivalents (1 ) (1,662 ) (218 ) 1,781 (100 ) Cash and cash equivalents at beginning of year 1 2,524 793 (2,562 ) 756 Cash and cash equivalents at end of year $ — $ 862 $ 575 $ (781 ) $ 656 Condensed Consolidating Statement of Cash Flows Year Ended December 31, 2017 (millions) Aon plc Aon Corporation Other Non-Guarantor Subsidiaries Consolidating Adjustments Consolidated Cash flows from operating activities Cash provided by operating activities - continuing operations $ 2,787 $ 503 $ 2,010 $ (4,631 ) $ 669 Cash provided by operating activities - discontinued operations — — 65 — 65 Cash provided by operating activities 2,787 503 2,075 (4,631 ) 734 Cash flows from investing activities Proceeds from investments 224 587 582 (1,325 ) 68 Payments for investments (261 ) (29 ) (576 ) 802 (64 ) Net sales (purchases) of short-term investments — non-fiduciary — (215 ) (17 ) — (232 ) Acquisition of businesses, net of cash acquired — — (1,029 ) — (1,029 ) Sale of businesses, net of cash sold — — 4,246 — 4,246 Capital expenditures — — (183 ) — (183 ) Cash provided by (used for) investing activities - continuing operations (37 ) 343 3,023 (523 ) 2,806 Cash used for investing activities - discontinued operations — — (19 ) — (19 ) Cash provided by (used for) investing activities (37 ) 343 3,004 (523 ) 2,787 Cash flows from financing activities Share repurchase (2,399 ) — — — (2,399 ) Advances from (to) affiliates 426 95 (4,975 ) 4,454 — Issuance of shares for employee benefit plans (121 ) — — — (121 ) Issuance of debt 544 1,100 10 — 1,654 Repayment of debt (835 ) (1,150 ) (14 ) — (1,999 ) Cash dividends to shareholders (364 ) — — — (364 ) Noncontrolling interests and other financing activities — — (36 ) — (36 ) Cash provided by (used for) financing activities - continuing operations (2,749 ) 45 (5,015 ) 4,454 (3,265 ) Cash used for financing activities - discontinued operations — — — — — Cash provided by (used for) financing activities (2,749 ) 45 (5,015 ) 4,454 (3,265 ) Effect of exchange rate changes on cash and cash equivalents — — 69 — 69 Net increase in cash and cash equivalents 1 891 133 (700 ) 325 Cash and cash equivalents at beginning of year (1) — 1,633 660 (1,862 ) 431 Cash and cash equivalents at end of year $ 1 $ 2,524 $ 793 $ (2,562 ) $ 756 (1) Includes $5 million of discontinued operations at December 31, 2016 .

Quarterly Financial Data (Una_2

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data	Selected quarterly financial data for the years ended December 31, 2019 and 2018 are as follows (in millions, except per share data): 1Q 2Q 3Q 4Q 2019 Income Statement Data Total revenue $ 3,143 $ 2,606 $ 2,379 $ 2,885 $ 11,013 Operating income 872 413 360 524 2,169 Net income from continuing operations 676 287 229 382 1,574 Income from discontinued operations, net of tax — — (1 ) — (1 ) Net income 676 287 228 382 1,573 Less: Net income attributable to noncontrolling interests 17 10 6 8 41 Net income attributable to Aon shareholders $ 659 $ 277 $ 222 $ 374 $ 1,532 Per Share Data Basic net income per share attributable to Aon shareholders Continuing operations $ 2.72 $ 1.15 $ 0.94 $ 1.59 $ 6.42 Discontinued operations — — — — — Net income $ 2.72 $ 1.15 $ 0.94 $ 1.59 $ 6.42 Diluted net income per share attributable to Aon shareholders Continuing operations $ 2.70 $ 1.14 $ 0.93 $ 1.58 $ 6.37 Discontinued operations — — — — — Net income $ 2.70 $ 1.14 $ 0.93 $ 1.58 $ 6.37 1Q 2Q 3Q 4Q 2018 Income Statement Data Total revenue $ 3,090 $ 2,561 $ 2,349 $ 2,770 $ 10,770 Operating income 799 (16 ) 262 499 1,544 Net income from continuing operations 604 57 155 284 1,100 Income from discontinued operations, net of tax 6 1 (2 ) 69 74 Net income 610 58 153 353 1,174 Less: Net income attributable to noncontrolling interests 16 10 6 8 40 Net income attributable to Aon shareholders $ 594 $ 48 $ 147 $ 345 $ 1,134 Per Share Data Basic net income per share attributable to Aon shareholders Continuing operations $ 2.37 $ 0.19 $ 0.61 $ 1.14 $ 4.32 Discontinued operations 0.02 0.01 (0.01 ) 0.28 0.30 Net income $ 2.39 $ 0.20 $ 0.60 $ 1.42 $ 4.62 Diluted net income per share attributable to Aon shareholders Continuing operations $ 2.35 $ 0.19 $ 0.61 $ 1.13 $ 4.29 Discontinued operations 0.02 — (0.01 ) 0.28 0.30 Net income $ 2.37 $ 0.19 $ 0.60 $ 1.41 $ 4.59

Summary of Significant Accoun_4

Summary of Significant Accounting Principles and Practices - Additional Information (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019GBP (£)	Jan. 01, 2020USD ($)	Dec. 31, 2019USD ($)	Jan. 01, 2019USD ($)	Dec. 31, 2018GBP (£)	Dec. 31, 2018USD ($)	Jan. 01, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Cash and Cash Equivalents
Cash and cash equivalents and short-term investments			$ 928			$ 828
Restricted cash and investments, current			110			91
Operating funds in U.K.	£ 42.7		55.5		£ 42.7	53.9
Fiduciary Assets and Liabilities
Premium trust balances			5,200			3,900
Allowance for Doubtful Accounts
Allowance for doubtful accounts			70			$ 62		$ 59	$ 56
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Operating lease right-of-use assets			929
Present value of lease liabilities			$ 1,130
Adoption of new accounting guidance							$ 492
Buildings
Fixed Assets
Fixed assets, original life, weighted-average	35 years
Automobiles
Fixed Assets
Fixed assets, original life, weighted-average	6 years
Minimum | Software
Fixed Assets
Fixed assets, original life, weighted-average	4 years
Minimum | Furniture, fixtures and equipment
Fixed Assets
Fixed assets, original life, weighted-average	4 years
Minimum | Computer equipment
Fixed Assets
Fixed assets, original life, weighted-average	4 years
Maximum | Software
Fixed Assets
Fixed assets, original life, weighted-average	7 years
Maximum | Leasehold improvements
Fixed Assets
Fixed assets, original life, weighted-average	10 years
Maximum | Furniture, fixtures and equipment
Fixed Assets
Fixed assets, original life, weighted-average	10 years
Maximum | Computer equipment
Fixed Assets
Fixed assets, original life, weighted-average	6 years
Accounting Standards Update 2016-02
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Operating lease right-of-use assets				$ 1,100
Present value of lease liabilities				$ 1,300
Accounting Standards Update 2014-09
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance							$ 507
Accounting Standards Update 2016-13 | Subsequent Event
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Adoption of new accounting guidance		$ 10
Tradenames | Minimum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	1 year
Tradenames | Maximum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	3 years
Customer related and contract based | Minimum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	7 years
Customer related and contract based | Maximum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	20 years
Technology | Minimum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	5 years
Technology | Maximum
Goodwill and Intangible Assets
Useful life of finite-lived intangible assets	7 years

Summary of Significant Accoun_5

Summary of Significant Accounting Principles and Practices - Schedule of Changes to Balance Sheet for New Accounting Standards (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets
Operating lease right-of-use assets	$ 929
Other non-current assets	570		$ 448
Liabilities
Other current liabilities	1,086		936
Non-current operating lease liabilities	944
Other non-current liabilities	$ 877		1,097
Operating Lease Right Of Use Assets
Assets
Operating lease right-of-use assets		$ 1,021
Other Noncurrent Assets
Assets
Other non-current assets		526	448
Other Current Liabilities
Liabilities
Other current liabilities		1,155	936
Other Noncurrent Operating Lease Liabilities
Liabilities
Non-current operating lease liabilities		1,014
Other Noncurrent Liabilities
Liabilities
Other non-current liabilities		963	$ 1,097
Accounting Standards Update 2016-02
Assets
Operating lease right-of-use assets		1,100
Accounting Standards Update 2016-02 | Operating Lease Right Of Use Assets
Assets
Operating lease right-of-use assets		1,021
Accounting Standards Update 2016-02 | Other Noncurrent Assets
Assets
Other non-current assets		78
Accounting Standards Update 2016-02 | Other Current Liabilities
Liabilities
Other current liabilities		219
Accounting Standards Update 2016-02 | Other Noncurrent Operating Lease Liabilities
Liabilities
Non-current operating lease liabilities		1,014
Accounting Standards Update 2016-02 | Other Noncurrent Liabilities
Liabilities
Other non-current liabilities		$ (134)

Revenue from Contracts with C_3

Revenue from Contracts with Customers - Disaggregation of Revenue (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
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,885	$ 2,379	$ 2,606	$ 3,143	$ 2,770	$ 2,349	$ 2,561	$ 3,090	$ 11,013	$ 10,770	$ 9,998
United States
Disaggregation of Revenue [Line Items]
Total revenue									5,016	4,677	4,425
Americas other than U.S.
Disaggregation of Revenue [Line Items]
Total revenue									919	940	976
U.K.
Disaggregation of Revenue [Line Items]
Total revenue									1,502	1,555	1,436
Europe, Middle East, & Africa other than United Kingdom
Disaggregation of Revenue [Line Items]
Total revenue									2,338	2,413	2,025
Asia Pacific
Disaggregation of Revenue [Line Items]
Total revenue									1,238	1,185	1,136
Commercial Risk Solutions
Disaggregation of Revenue [Line Items]
Total revenue									4,673	4,652	4,169
Reinsurance Solutions
Disaggregation of Revenue [Line Items]
Total revenue									1,686	1,563	1,429
Retirement Solutions
Disaggregation of Revenue [Line Items]
Total revenue									1,817	1,865	1,755
Health Solutions
Disaggregation of Revenue [Line Items]
Total revenue									1,667	1,596	1,515
Data & Analytic Services
Disaggregation of Revenue [Line Items]
Total revenue									1,184	1,105	1,140
Elimination
Disaggregation of Revenue [Line Items]
Total revenue									$ (14)	$ (11)	$ (10)

Revenue from Contracts with C_4

Revenue from Contracts with Customers - Contract Assets Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Capitalized Cost To Fulfill Customer Contracts
Change in Capitalized Contract Costs
Balance at beginning of period	$ 329	$ 298
Additions	1,453	1,504
Amortization	(1,447)	(1,465)
Impairment	0	0
Foreign currency translation and other	0	(8)
Balance at end of period	335	329
Capitalized Cost To Obtain Customer Contracts
Change in Capitalized Contract Costs
Balance at beginning of period	156	145
Additions	58	53
Amortization	(44)	(41)
Impairment	0	0
Foreign currency translation and other	1	(1)
Balance at end of period	$ 171	$ 156

Other Financial Data - Schedule

Other Financial Data - Schedule of Other Income (Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Financial Data [Abstract]
Foreign currency remeasurement	$ 9	$ 25	$ (37)
Disposal of businesses	13	(6)	(16)
Pension and other postretirement	9	1	(86)
Equity earnings	4	4	12
Financial instruments	(34)	(49)	2
Other income (expense)	$ 1	$ (25)	$ (125)

Other Financial Data - Schedu_2

Other Financial Data - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 62	$ 59	$ 56
Provision	27	24	18
Accounts written off, net of recoveries	(19)	(25)	(18)
Foreign currency translation and other	0	4	3
Balance at end of period	$ 70	$ 62	$ 59

Other Financial Data - Schedu_3

Other Financial Data - Schedule of Other Current Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Other Financial Data [Abstract]
Costs to fulfill contracts with customers	$ 335	$ 329
Taxes receivable	88	113
Prepaid expenses	97	97
Receivables from the Divested Business	4	12
Other	78	67
Total	$ 602	$ 618

Other Financial Data - Componen

Other Financial Data - Components of Fixed Assets, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fixed Assets, net
Fixed assets, gross	$ 1,780	$ 1,733
Less: Accumulated depreciation	1,159	1,145
Fixed assets, net	621	588
Depreciation of fixed assets	172	176	$ 187
Software
Fixed Assets, net
Fixed assets, gross	727	693
Leasehold improvements
Fixed Assets, net
Fixed assets, gross	358	334
Computer equipment
Fixed Assets, net
Fixed assets, gross	250	279
Furniture, fixtures and equipment
Fixed Assets, net
Fixed assets, gross	225	228
Construction in progress
Fixed Assets, net
Fixed assets, gross	183	154
Other
Fixed Assets, net
Fixed assets, gross	$ 37	$ 45

Other Financial Data - Schedu_4

Other Financial Data - Schedule of Other Non-current Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Other Financial Data [Abstract]
Costs to obtain contracts with customers	$ 171	$ 156
Investments	53	54
Leases	100
Taxes receivable	102	100
Other	144	138
Total	$ 570	$ 448

Other Financial Data - Schedu_5

Other Financial Data - Schedule of Other Current Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Other Financial Data [Abstract]
Deferred revenue	$ 270	$ 251
Taxes payable	93	83
Leases	210
Other	513	602
Total	1,086	936
Deferred revenue recognized	$ 532	$ 487

Other Financial Data - Schedu_6

Other Financial Data - Schedule of Other Non-current Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Other Financial Data [Abstract]
Taxes payable	$ 525	$ 585
Leases	76
Leases		169
Compensation and benefits	49	56
Deferred revenue	62	65
Other	165	222
Total other non-current liabilities	877	1,097
Transition tax for accumulated foreign earnings, noncurrent	$ 145	$ 240

Discontinued Operations - Narra

Discontinued Operations - Narrative (Details)	May 01, 2017USD ($)agreement	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and cash equivalents of discontinued operations			$ 5,000,000
Tempo Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of commercial agreements | agreement	2
Tempo Business | Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase price, before adjustment	$ 4,300,000,000
Purchase price	4,200,000,000
Depreciation of fixed assets		$ 8,000,000
Intangible asset amortization		11,000,000
Cash and cash equivalents of discontinued operations		$ 0
Maximum | Tempo Business | Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Deferred consideration	$ 500,000,000

Discontinued Operations - Incom

Discontinued Operations - Income Statement (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
Expenses
Net income (loss) from discontinued operations	$ 0	$ (1)	$ 0	$ 0	$ 69	$ (2)	$ 1	$ 6	$ (1)	$ 74	$ 828
Tempo Business | Discontinued Operations
Revenue
Total revenue									0	0	698
Expenses
Total operating expenses									2	12	656
Operating income from discontinued operations									(2)	(12)	42
Other income									0	0	10
Income (loss) from discontinued operations before income taxes									(2)	(12)	52
Income tax expense (benefit)									(1)	(4)	3
Net income (loss) from discontinued operations, excluding gain									(1)	(8)	49
Gain on sale of discontinued operations, net of tax									0	82	779
Net income (loss) from discontinued operations									$ (1)	$ 74	$ 828

Restructuring - Narrative (Deta

Restructuring - Narrative (Details) - 2017 Plan $ in Millions	36 Months Ended
	Dec. 31, 2019USD ($)job_elimination
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	$ 1,433
Number of positions eliminated | job_elimination	5,832
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	$ 619
Technology rationalization
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	119
Lease consolidation
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	69
Asset impairments
Restructuring Cost and Reserve [Line Items]
Completed Plan Total	$ 53

Restructuring - Schedule of Res

Restructuring - Schedule of Restructuring and Related Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost			$ 497
2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	$ 451	$ 485
Completed Plan Total	1,433
Workforce reduction | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	205	115	299
Completed Plan Total	619
Technology rationalization | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	39	47	33
Completed Plan Total	119
Incurred contract termination costs	0	5	1
Lease consolidation | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	33	28	8
Completed Plan Total	69
Incurred contract termination costs	33	25	8
Asset impairments | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	14	13	26
Completed Plan Total	53
Other Restructuring | 2017 Plan
Restructuring Cost and Reserve [Line Items]
Incurred restructuring cost	160	282	131
Completed Plan Total	573
Incurred contract termination costs	$ 13	$ 85	$ 3

Restructuring - Schedule of R_2

Restructuring - Schedule of Restructuring Reserve (Details) - 2017 Plan $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, beginning balance	$ 201
Expensed	418
Cash payments	(415)
Foreign currency translation and other	0
Restructuring reserve, ending balance	$ 204

Acquisitions and Dispositions_3

Acquisitions and Dispositions of Businesses - Completed Acquisitions Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018
Business Acquisition [Line Items]
Number of business acquired under business combination	3	8
Weighted average useful life	7 years
Revenues from acquisitions included in the Company's Consolidated Statement of Income	$ 7
Other general expense
Business Acquisition [Line Items]
Acquisition related costs	$ 1

Acquisitions and Dispositions_4

Acquisitions and Dispositions of Businesses - Schedule of Net Assets Acquired (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets acquired
Goodwill	$ 8,165	$ 8,171	$ 8,358
2018 and 2019 Acquisitions
Consideration transferred
Cash	42
Deferred and contingent consideration	8
Aggregate consideration transferred	50
Assets acquired
Cash and cash equivalents	4
Receivables, net	0
Goodwill	34
Intangible assets, net	22
Fixed assets, net	1
Other assets	13
Total assets acquired	74
Liabilities assumed
Current liabilities	18
Other non-current liabilities	6
Total liabilities assumed	24
Net assets acquired	$ 50

Acquisitions and Dispositions_5

Acquisitions and Dispositions of Businesses - Completed Dispositions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gains (losses) on disposal of businesses	$ 13	$ (6)	$ (16)
Disposal Group, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of dispositions	8	4	9
Amortization and Impairment of Intangible Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-cash impairment charge of disposition			$ 380
Amortization and Impairment of Intangible Assets | Disposal Group, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Non-cash impairment charge of disposition		$ 176

Goodwill and Other Intangible_3

Goodwill and Other Intangible Assets - Schedule of changes in the net carrying amount of goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Changes in the net carrying amount of goodwill (in millions)
Balance at the beginning of the period	$ 8,171	$ 8,358
Goodwill related to current year acquisitions	34	38
Goodwill related to disposals	(11)	(2)
Goodwill related to prior year acquisitions	2	4
Foreign currency translation	(31)	(227)
Balance at the end of the period	$ 8,165	$ 8,171

Goodwill and Other Intangible_4

Goodwill and Other Intangible Assets - Schedule of other intangible assets by asset class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 3,673	$ 3,658
Accumulated Amortization and Impairment	2,890	2,509
Net Carrying Amount	783	1,149
Amortization and impairment of intangible assets	392	593	$ 704
Customer related and contract based
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2,264	2,240
Accumulated Amortization and Impairment	1,600	1,444
Net Carrying Amount	664	796
Tradenames
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,029	1,027
Accumulated Amortization and Impairment	956	740
Net Carrying Amount	73	287
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	380	391
Accumulated Amortization and Impairment	334	325
Net Carrying Amount	$ 46	$ 66

Goodwill and Other Intangible_5

Goodwill and Other Intangible Assets - Schedule of estimated future amortization expense on intangible assets (Details) $ in Millions	Dec. 31, 2019USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2020	$ 223
2021	127
2022	86
2023	76
2024	59
Thereafter	212
Total future amortization of intangible assets	$ 783

Debt - Summary of outstanding d

Debt - Summary of outstanding debt (Details)	Dec. 31, 2019USD ($)	Dec. 31, 2019EUR (€)	Nov. 25, 2019USD ($)	Sep. 30, 2019USD ($)	May 02, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 03, 2018USD ($)
Debt Instrument [Line Items]
Total debt	$ 7,339,000,000					$ 6,244,000,000
Less: Short-term debt and current portion of long-term debt	712,000,000					251,000,000
Long-term debt	6,627,000,000					5,993,000,000
Other
Debt Instrument [Line Items]
Total debt	1,000,000					1,000,000
5.00% Senior Notes due September 2020 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 600,000,000					599,000,000
Interest rate on debt	5.00%	5.00%		5.00%
Debt face value				$ 600,000,000
2.80% Senior Notes due March 2021
Debt Instrument [Line Items]
Interest rate on debt	2.80%	2.80%
2.80% Senior Notes due March 2021 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 399,000,000					398,000,000
Interest rate on debt	2.80%	2.80%
2.20% Senior Notes due November 2022 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 497,000,000					0
Interest rate on debt	2.20%	2.20%
Debt face value			$ 500,000,000
4.00% Senior Notes due November 2023 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 348,000,000					348,000,000
Interest rate on debt	4.00%	4.00%
3.50% Senior Notes due June 2024 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 597,000,000					596,000,000
Interest rate on debt	3.50%	3.50%
3.875% Senior Notes due December 2025
Debt Instrument [Line Items]
Interest rate on debt	3.875%	3.875%
3.875% Senior Notes due December 2025 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 746,000,000					746,000,000
Interest rate on debt	3.875%	3.875%
2.875% Senior Notes due May 2026 (EUR 500M) | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 550,000,000					562,000,000
Interest rate on debt	2.875%	2.875%
Debt face value | €		€ 500,000,000
8.205% Junior Subordinated Notes due January 2027 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 521,000,000					521,000,000
Interest rate on debt	8.205%	8.205%
4.50% Senior Notes due December 2028 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 346,000,000					347,000,000
Interest rate on debt	4.50%	4.50%					4.50%
Debt face value							$ 350,000,000
3.75% Senior Notes due May 2029 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 744,000,000					0
Interest rate on debt	3.75%	3.75%			3.75%
Debt face value					$ 750,000,000
6.25% Senior Notes due September 2040 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 296,000,000					296,000,000
Interest rate on debt	6.25%	6.25%
4.25% Senior Notes due December 2042 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 199,000,000					198,000,000
Interest rate on debt	4.25%	4.25%
4.45% Senior Notes due May 2043 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 246,000,000					246,000,000
Interest rate on debt	4.45%	4.45%
4.60% Senior Notes due June 2044 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 544,000,000					544,000,000
Interest rate on debt	4.60%	4.60%
4.75% Senior Notes due May 2045
Debt Instrument [Line Items]
Interest rate on debt	4.75%	4.75%
4.75% Senior Notes due May 2045 | Senior Notes
Debt Instrument [Line Items]
Total debt	$ 593,000,000					592,000,000
Interest rate on debt	4.75%	4.75%
Commercial paper
Debt Instrument [Line Items]
Total debt	$ 112,000,000					$ 250,000,000

Debt - Narrative (Details)

Debt - Narrative (Details)	12 Months Ended
	Dec. 31, 2019USD ($)credit_facility	Dec. 31, 2019EUR (€)	Nov. 25, 2019USD ($)	Sep. 30, 2019USD ($)	May 02, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 03, 2018USD ($)
Debt Instrument [Line Items]
Short-term debt and current portion of long-term debt	$ 712,000,000					$ 251,000,000
Number of committed credit facilities | credit_facility	2
Senior Notes | 2.20% Senior Notes due November 2022
Debt Instrument [Line Items]
Debt face value			$ 500,000,000
Interest rate on debt	2.20%	2.20%
Senior Notes | 5.00% Senior Notes due September 2020
Debt Instrument [Line Items]
Debt face value				$ 600,000,000
Interest rate on debt	5.00%	5.00%		5.00%
Senior Notes | 3.75% Senior Notes due May 2029
Debt Instrument [Line Items]
Debt face value					$ 750,000,000
Interest rate on debt	3.75%	3.75%			3.75%
Senior Notes | 4.50% Senior Notes due December 2028
Debt Instrument [Line Items]
Debt face value							$ 350,000,000
Interest rate on debt	4.50%	4.50%					4.50%
Credit Facility Expiring February 2022
Debt Instrument [Line Items]
New credit and loan facility	$ 900,000,000
Line of credit oustanding	0
Credit Facility Expiring October 2023
Debt Instrument [Line Items]
Line of credit oustanding	0
Credit Facility Expiring October 2023 | Line of Credit
Debt Instrument [Line Items]
New credit and loan facility	400,000,000
Commercial paper
Debt Instrument [Line Items]
Current borrowing capacity	1,300,000,000
United States | Commercial paper
Debt Instrument [Line Items]
New credit and loan facility	$ 600,000,000
Europe | Commercial paper
Debt Instrument [Line Items]
New credit and loan facility | €		€ 525,000,000

Debt - Repayments of long-term

Debt - Repayments of long-term debt (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Debt Disclosure [Abstract]
2020	$ 712
2021	400
2022	500
2023	350
2024	600
Thereafter	4,882
Total Repayments	7,444
Unamortized discounts, premiums, and debt issuance costs	(105)
Total Debt	$ 7,339

Debt - Schedule of Commercial P

Debt - Schedule of Commercial Paper (Details) - Commercial paper - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 112	$ 250
Weighted average commercial paper outstanding	$ 511	$ 580
Weighted average interest rate of commercial paper outstanding	0.27%	0.84%

Lease Commitments - Assets and

Lease Commitments - Assets and Liabilities of Lessee (Details) $ in Millions	Dec. 31, 2019USD ($)
Assets
Operating lease assets	$ 929
Finance lease assets	100
Total lease assets	1,029
Current lease liabilities
Operating	186
Finance	24
Non-current lease liabilities
Operating	944
Finance	76
Total lease liabilities	$ 1,230

Lease Commitments - Lease Costs

Lease Commitments - Lease Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating lease cost	$ 234
Finance lease costs
Amortization of leased assets	26
Interest on lease liabilities	2
Variable lease cost	60
Short-term lease cost	5
Sublease income	(32)
Net lease cost	$ 295

Lease Commitments - Lease Terms

Lease Commitments - Lease Terms and Assumptions (Details)	Dec. 31, 2019
Weighted average remaining lease term (years)
Operating leases	7 years 10 months 24 days
Finance leases	4 years 4 months 24 days
Weighted average discount rate
Operating leases	3.20%
Finance leases	2.00%

Lease Commitments - Cash Flow o

Lease Commitments - Cash Flow of Lessee (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows for operating leases	$ 264
Financing cash flows for finance leases	17
Non-cash related activities
ROU assets obtained in exchange for new operating lease liabilities	155
ROU assets obtained in exchange for new finance lease liabilities	48
Operating lease ROU asset expense	195
Changes in Non-current operating lease liabilities	$ (70)

Lease Commitments - Lease Matur

Lease Commitments - Lease Maturity (Details) $ in Millions	Dec. 31, 2019USD ($)
Operating Leases
2020	$ 208
2021	202
2022	177
2023	141
2024	113
Thereafter	432
Total undiscounted future minimum lease payments	1,273
Less: Imputed interest	(143)
Present value of lease liabilities	1,130
Finance Leases
2020	26
2021	26
2022	20
2023	18
2024	15
Thereafter	0
Less: Imputed interest	105
Less: Imputed interest	(5)
Present value of lease liabilities	100
Total
2020	234
2021	228
2022	197
2023	159
2024	128
Thereafter	432
Total undiscounted future minimum lease payments	1,378
Less: Imputed interest	(148)
Present value of lease liabilities	$ 1,230

Lease Commitments - Lease Mat_2

Lease Commitments - Lease Maturity Prior to Topic 842 (Details) $ in Millions	Dec. 31, 2018USD ($)
Gross rental commitments
2020	$ 303
2021	253
2022	221
2023	182
2024	148
Thereafter	472
Total minimum payments required	1,579
Rentals from subleases
2020	(34)
2021	(30)
2022	(30)
2023	(30)
2024	(12)
Thereafter	(5)
Total minimum payments required	(141)
Net rental commitments
2020	269
2021	223
2022	191
2023	152
2024	136
Thereafter	467
Total minimum payments required	$ 1,438

Income Taxes - Income from cont

Income Taxes - Income from continuing operations before income tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income before income taxes:
Income before income taxes:	$ 1,871	$ 1,246	$ 685
Current:
Total current tax expense	333	371	268
Deferred tax expense (benefit):
Total deferred tax benefit	(36)	(225)	(18)
Total income tax expense	297	146	250
U.K.
Income before income taxes:
Income before income taxes:	228	(240)	(420)
Current:
Total current tax expense	20	21	1
Deferred tax expense (benefit):
Total deferred tax benefit	35	19	(5)
U.S. federal
Income before income taxes:
Income before income taxes:	(219)	(601)	(765)
Current:
Total current tax expense	22	101	48
Deferred tax expense (benefit):
Total deferred tax benefit	(20)	(165)	12
U.S. state and local
Current:
Total current tax expense	41	35	18
Deferred tax expense (benefit):
Total deferred tax benefit	(27)	(56)	(35)
Other
Income before income taxes:
Income before income taxes:	1,862	2,087	1,870
Current:
Total current tax expense	250	214	201
Deferred tax expense (benefit):
Total deferred tax benefit	$ (24)	$ (23)	$ 10

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory tax rate	19.00%	19.00%	19.30%
Increase in valuation allowance	$ 29
Unremitted earnings	28	$ 30
Benefit realized from tax holiday granted	$ 90	$ 77	$ 45
Earnings per share impact of tax holiday (in dollars per share)	$ 0.37	$ 0.31	$ 0.17
Unrecognized tax benefits that would impact effective tax rate	$ 248	$ 228	$ 219
Accrued potential interest and penalties	24	22	11
Liability recorded for interest and penalties	$ 99	$ 77	$ 55

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of the income tax provisions based on the U.S. statutory corporate tax rate to the provisions reflected in the Consolidated Financial Statements (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the income tax provisions based on the statutory corporate tax rate to the provisions reflected in the Consolidated Financial Statements
Statutory tax rate	19.00%	19.00%	19.30%
U.S. state income taxes, net of U.S. federal benefit	0.50%	(0.40%)	(1.50%)
Taxes on international operations	(6.00%)	(7.30%)	(30.30%)
Nondeductible expenses	1.60%	2.70%	3.40%
Adjustments to prior year tax requirements	0.10%	0.90%	2.00%
Adjustments to valuation allowances	1.80%	3.80%	(1.80%)
Change in uncertain tax positions	2.20%	0.90%	1.60%
Excess tax benefits related to shared based compensation	(2.80%)	(3.60%)	(8.00%)
U.S. Tax Reform impact	(0.003)	0.071	0.512
Loss on disposition	0.00%	(10.20%)	0.00%
Other — net	(0.20%)	(1.20%)	0.60%
Effective tax rate	15.90%	11.70%	36.50%

Income Taxes - Components of Ao

Income Taxes - Components of Aon's deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss, capital loss, interest, and tax credit carryforwards	$ 440	$ 563
Employee benefit plans	373	351
Lease liabilities	247
Accrued interest	116	0
Other accrued expenses	68	98
Investment basis differences	28	28
Deferred revenue	24	29
Lease and service guarantees	3	5
Other	57	46
Total	1,356	1,120
Valuation allowance on deferred tax assets	(200)	(171)
Total	1,156	949
Deferred tax liabilities:
Intangibles and property, plant and equipment	(251)	(310)
Lease right-of-use asset	(219)
Deferred costs	(128)	(143)
Unremitted earnings	(28)	(30)
Unrealized foreign exchange gains	(26)	(26)
Other accrued expenses	(25)	(36)
Other	(33)	(24)
Total	(710)	(569)
Net deferred tax asset	$ 446	$ 380

Income Taxes - Deferred income

Income Taxes - Deferred income taxes (assets and liabilities netted by jurisdiction) as classified in the Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Deferred tax assets — non-current	$ 645	$ 561
Deferred tax liabilities — non-current	(199)	(181)
Net deferred tax asset	$ 446	$ 380

Income Taxes Operating Loss Car

Income Taxes Operating Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
U.K.
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 438	$ 541
Interest carryforwards	411	400
Interest carryforwards	203	53
U.S. Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	1	2
Interest carryforwards	112	367
Interest carryforwards	355	424
U.S. State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	376	315
Interest carryforwards	123	221
Interest carryforwards	172	227
Other
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	308	369
Interest carryforwards	29	30
Interest carryforwards	$ 159	$ 186

Income Taxes - Reconciliation_2

Income Taxes - Reconciliation of the beginning and ending amount of unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the Company's beginning and ending amount of unrecognized tax benefits
Balance at the beginning of the period	$ 279	$ 280
Additions based on tax positions related to the current year	23	18
Additions for tax positions of prior years	12	10
Reductions for tax positions of prior years	(5)	(24)
Settlements	(5)	0
Business combinations	0	1
Lapse of statute of limitations	(5)	(6)
Foreign currency translation, increase	0	0
Balance at the end of the period	$ 299	$ 279

Shareholders' Equity - Distribu

Shareholders' Equity - Distributable Reserves Narrative (Details) - USD ($) $ in Billions	Dec. 31, 2019	Jul. 16, 2019	Dec. 31, 2018
Equity [Abstract]
Distributable reserves available amount	$ 32.4	$ 31	$ 2.2

Shareholders' Equity - Ordinary

Shareholders' Equity - Ordinary Shares (Details) - USD ($) $ / shares in Units, shares in Millions	12 Months Ended		21 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Jun. 30, 2017	Nov. 30, 2014	Apr. 30, 2012
Common Stock Programs
Share repurchase authorization limit				$ 15,000,000,000
Number of shares repurchased (in shares)			128.7
Costs recorded to retained earnings
Total repurchase cost			$ 13,000,000,000
Share Repurchase Program of 2012
Common Stock Programs
Share repurchase authorization limit						$ 5,000,000,000
Number of shares repurchased (in shares)	10.5	10
Average price per share (in dollars per share)	$ 186.33	$ 143.94
Costs recorded to retained earnings
Total repurchase cost	$ 1,950,000,000	$ 1,447,000,000
Additional associated costs	10,000,000	7,000,000
Total costs recorded to retained earnings	1,960,000,000	$ 1,454,000,000
Share Repurchase Program of 2014
Common Stock Programs
Share repurchase authorization limit					$ 5,000,000,000
Share Repurchase Program of 2017
Common Stock Programs
Share repurchase authorization limit				$ 5,000,000,000
Costs recorded to retained earnings
Remaining authorized repurchase amount	$ 2,000,000,000		$ 2,000,000,000

Shareholders' Equity - Schedule

Shareholders' Equity - Schedule of weighted average shares outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Basic weighted average ordinary shares outstanding (in shares)	238,600,000	245,200,000	258,500,000
Dilutive effect of potentially issuable shares (in shares)	2,000,000	1,800,000	2,200,000
Diluted weighted average ordinary shares outstanding (in shares)	240,600,000	247,000,000	260,700,000
Antidilutive securities excluded from computation of earnings per share (in shares)	0	0	0

Shareholders' Equity - Dividend

Shareholders' Equity - Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Cash dividends to shareholders	$ 410	$ 382	$ 364
Dividends paid per share (in dollars per share)	$ 1.72	$ 1.56	$ 1.41

Shareholders' Equity - Componen

Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 4,219	$ 4,648	$ 5,581
Adoption of new accounting guidance				$ 492
Beginning Balance, adjusted				5,140
Other comprehensive income (loss) before reclassifications	(282)	(576)	195
Tax benefit (expense)	58	43	47
Other comprehensive income (loss) before reclassifications, net	(224)	(533)	242
Amounts reclassified from accumulated other comprehensive income (loss)	123	157	227
Tax benefit (expense)	(23)	(36)	(53)
Amounts reclassified from accumulated other comprehensive income (loss), net	100	121	174
Total other comprehensive income (loss) attributable to Aon shareholders	(124)	(412)	416
Ending Balance	3,449	4,219	4,648
Change in Fair Value of Investments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(15)	(25)	(37)
Adoption of new accounting guidance				(1)
Beginning Balance, adjusted				(26)
Other comprehensive income (loss) before reclassifications	(10)	(15)	18
Tax benefit (expense)	1	18	(3)
Other comprehensive income (loss) before reclassifications, net	(9)	3	15
Amounts reclassified from accumulated other comprehensive income (loss)	14	11	(2)
Tax benefit (expense)	(2)	(3)	(1)
Amounts reclassified from accumulated other comprehensive income (loss), net	12	8	(3)
Total other comprehensive income (loss) attributable to Aon shareholders	3	11	12
Ending Balance	(12)	(15)	(25)
Foreign Currency Translation Adjustments
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(1,319)	(879)	(1,264)
Adoption of new accounting guidance				0
Beginning Balance, adjusted				(879)
Other comprehensive income (loss) before reclassifications	15	(437)	397
Tax benefit (expense)	(1)	(3)	(5)
Other comprehensive income (loss) before reclassifications, net	14	(440)	392
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	(7)
Tax benefit (expense)	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss), net	0	0	(7)
Total other comprehensive income (loss) attributable to Aon shareholders	14	(440)	385
Ending Balance	(1,305)	(1,319)	(879)
Post-retirement Benefit Obligations
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,575)	(2,592)	(2,611)
Adoption of new accounting guidance				0
Beginning Balance, adjusted				(2,592)
Other comprehensive income (loss) before reclassifications	(287)	(124)	(220)
Tax benefit (expense)	58	28	55
Other comprehensive income (loss) before reclassifications, net	(229)	(96)	(165)
Amounts reclassified from accumulated other comprehensive income (loss)	109	146	236
Tax benefit (expense)	(21)	(33)	(52)
Amounts reclassified from accumulated other comprehensive income (loss), net	88	113	184
Total other comprehensive income (loss) attributable to Aon shareholders	(141)	17	19
Ending Balance	(2,716)	(2,575)	(2,592)
AOCI attributable to parent
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,909)	(3,496)	(3,912)
Adoption of new accounting guidance				(1)
Beginning Balance, adjusted				$ (3,497)
Ending Balance	$ (4,033)	$ (3,909)	$ (3,496)

Employee Benefits - Schedule of

Employee Benefits - Schedule of expense recognized in Compensation and benefit in the Consolidated Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expense recognized for defined contribution savings plans	$ 164	$ 168	$ 173
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expense recognized for defined contribution savings plans	98	98	105
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expense recognized for defined contribution savings plans	41	45	43
Netherlands and Canada
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expense recognized for defined contribution savings plans	$ 25	$ 25	$ 25

Employee Benefits - Pension Pla

Employee Benefits - Pension Plans Narrative (Details) £ in Millions, $ in Millions	1 Months Ended	3 Months Ended				12 Months Ended
	Sep. 30, 2018USD ($)	Dec. 31, 2018GBP (£)	Dec. 31, 2018USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2017USD ($)	Dec. 31, 2019GBP (£)	Dec. 31, 2019USD ($)	Dec. 31, 2018GBP (£)	Dec. 31, 2018USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2017USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percentage of the Company's projected benefit obligation							91.00%
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Voluntary contribution	$ 100
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO			$ 2,900				$ 3,100		$ 2,900
Plans with ABO in excess of the fair value of plan assets, ABO			2,900				3,100		2,900
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets			1,800				2,000		1,800
Payment for settlements							0		0
Non-cash settlement charge							0		0		$ 0
Amounts in Accumulated other comprehensive income (loss) expected to be recognized as components of net periodic benefit cost in next fiscal year							(68)
Foreign Plan | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts in Accumulated other comprehensive income (loss) expected to be recognized as components of net periodic benefit cost in next fiscal year							(44)
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO			47				54		47
Plans with ABO in excess of the fair value of plan assets, ABO			47				54		47
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets			30				34		30
Plans with PBO in excess of plan assets, plan assets			30						30
Payment for settlements							22	£ 139	176	£ 371	496
Reduction of projected benefit obligation		£ 122	154	£ 325	$ 434
Non-cash settlement charge				£ 93	$ 125	£ 4	5	£ 28	37		125
Other | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Plans with PBO in excess of plan assets, PBO			1,200				1,400		1,200
Plans with ABO in excess of the fair value of plan assets, ABO			363				406		363
Plans with ABO in excess of the fair value of plan assets, fair value of plan assets			271				311		271
Plans with PBO in excess of plan assets, plan assets			$ 1,100				1,300		1,100
Payment for settlements							0		0
Non-cash settlement charge							$ 0		$ 0		$ 0

Employee Benefits - Reconciliat

Employee Benefits - Reconciliation of the changes in the benefit obligations and fair value of assets and a statement of the funded status (Details) - Pension Plan £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018GBP (£)	Dec. 31, 2018USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2017USD ($)
U.S.
Change in projected benefit obligation
Balance at the beginning of the Period	$ 2,877		$ 3,155
Service cost	0		0		$ 0
Interest cost	108		99		96
Plan amendment	0		0
Settlements	0		0
Actuarial loss (gain)	373		(221)
Benefit payments	(166)		(156)
Foreign currency impact	0		0
Balance at the end of the period	3,192		2,877		3,155
Accumulated benefit obligation at end of year	3,192		2,877
Change in fair value of plan assets
Balance at the beginning of the period	1,796		1,958
Actual return on plan assets	398		(141)
Employer contributions	38		135
Settlements	0		0
Benefit payments	(166)		(156)
Foreign currency impact	0		0
Balance at the end of the period	2,066		1,796		1,958
Market related value at end of year	1,969		1,981
Funded status	(1,126)		(1,081)
Unrecognized prior-service cost	1		3
Unrecognized loss	1,762		1,705
Net amount recognized	637		627
U.K.
Change in projected benefit obligation
Balance at the beginning of the Period	4,129		4,893
Service cost	0		0		0
Interest cost	109		109		123
Plan amendment	10		13
Settlements	(22)		(176)
Actuarial loss (gain)	594		(297)
Benefit payments	(168)		(160)
Foreign currency impact	127		(253)
Balance at the end of the period	4,779		4,129		4,893
Accumulated benefit obligation at end of year	4,779		4,129
Change in fair value of plan assets
Balance at the beginning of the period	5,225		5,906
Actual return on plan assets	687		(125)
Employer contributions	78		97
Settlements	(22)	£ (139)	(176)	£ (371)	(496)
Benefit payments	(168)		(160)
Foreign currency impact	159		(317)
Balance at the end of the period	5,959		5,225		5,906
Market related value at end of year	5,959		5,225
Funded status	1,180		1,096
Unrecognized prior-service cost	40		30
Unrecognized loss	1,204		1,106
Net amount recognized	2,424		2,232
Other
Change in projected benefit obligation
Balance at the beginning of the Period	1,271		1,401
Service cost	0		0		0
Interest cost	27		27		26
Plan amendment	0		0
Settlements	0		0
Actuarial loss (gain)	177		(47)
Benefit payments	(42)		(43)
Foreign currency impact	(8)		(67)
Balance at the end of the period	1,425		1,271		1,401
Accumulated benefit obligation at end of year	1,391		1,247
Change in fair value of plan assets
Balance at the beginning of the period	1,155		1,256
Actual return on plan assets	182		(19)
Employer contributions	19		20
Settlements	0		0
Benefit payments	(42)		(43)
Foreign currency impact	(11)		(59)
Balance at the end of the period	1,303		1,155		$ 1,256
Market related value at end of year	1,303		1,155
Funded status	(122)		(116)
Unrecognized prior-service cost	(6)		(7)
Unrecognized loss	460		440
Net amount recognized	$ 332		$ 317

Employee Benefits - Amounts rec

Employee Benefits - Amounts recognized in the Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	$ 1,216	$ 1,133
Pension Plan | U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	0	0
Accrued benefit liability - current	(50)	(46)
Accrued benefit liability - non-current	(1,076)	(1,035)
Accumulated other comprehensive loss	1,763	1,708
Net amount recognized	637	627
Pension Plan | U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	1,200	1,113
Accrued benefit liability - current	(1)	(1)
Accrued benefit liability - non-current	(19)	(16)
Accumulated other comprehensive loss	1,244	1,136
Net amount recognized	2,424	2,232
Pension Plan | Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Prepaid pension	0	0
Accrued benefit liability - current	(5)	(5)
Accrued benefit liability - non-current	(117)	(111)
Accumulated other comprehensive loss	454	433
Net amount recognized	$ 332	$ 317

Employee Benefits - Amounts r_2

Employee Benefits - Amounts recognized in Accumulated other comprehensive loss that have not yet been recognized (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
U.S.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	$ 1,762	$ 1,705
Prior service cost (income)	1	3
Net amount recognized	1,763	1,708
U.K.
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	1,204	1,106
Prior service cost (income)	40	30
Net amount recognized	1,244	1,136
Other
Amounts recognized in Accumulated other comprehensive loss unrecognized as components of net periodic benefit cost
Net loss	460	440
Prior service cost (income)	(6)	(7)
Net amount recognized	$ 454	$ 433

Employee Benefits - Components

Employee Benefits - Components of net periodic benefit cost (Details) - Pension Plan £ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017GBP (£)	Dec. 31, 2017USD ($)	Dec. 31, 2019GBP (£)	Dec. 31, 2019USD ($)	Dec. 31, 2018GBP (£)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
U.S.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost				$ 0		$ 0	$ 0
Interest cost				108		99	96
Expected return on plan assets, net of administration expenses				(136)		(144)	(140)
Amortization of prior-service cost				2		2	2
Amortization of net actuarial loss				53		59	50
Net periodic benefit (income) cost				27		16	8
Settlement expense				0		0	0
Total net periodic benefit cost (income)				27		16	8
U.K.
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost				0		0	0
Interest cost				109		109	123
Expected return on plan assets, net of administration expenses				(191)		(192)	(199)
Amortization of prior-service cost				1		1	1
Amortization of net actuarial loss				29		28	31
Net periodic benefit (income) cost				(52)		(54)	(44)
Settlement expense	£ 93	$ 125	£ 4	5	£ 28	37	125
Total net periodic benefit cost (income)				(47)		(17)	81
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost				0		0	0
Interest cost				27		27	26
Expected return on plan assets, net of administration expenses				(40)		(45)	(47)
Amortization of prior-service cost				0		0	0
Amortization of net actuarial loss				12		12	11
Net periodic benefit (income) cost				(1)		(6)	(10)
Settlement expense				0		0	0
Total net periodic benefit cost (income)				$ (1)		$ (6)	$ (10)

Employee Benefits - Weighted-av

Employee Benefits - Weighted-average assumptions used to determine future benefit obligations (Details)	Dec. 31, 2019	Dec. 31, 2018
U.S. | Pension Plan | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.72%	3.92%
U.S. | Pension Plan | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.17%	4.26%
U.K. | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.24%	3.73%
U.K. | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.74%	4.23%
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.09%	2.95%
Underlying price inflation	1.78%	1.88%
Other | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	1.00%	1.00%
Other | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.00%	3.00%
Other | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Underlying price inflation	2.00%	2.00%
Other | Pension Plan | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	0.90%	1.89%
Other | Pension Plan | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.10%	3.88%

Employee Benefits - Weighted-_2

Employee Benefits - Weighted-average assumptions used to determine the net periodic benefit cost (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets, net of administration expenses	7.05%	7.71%	7.88%
U.S. | Pension Plan | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.92%	3.27%	3.53%
U.S. | Pension Plan | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.26%	3.61%	4.11%
U.K. | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.73%	3.70%	3.70%
U.K. | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	4.23%	4.20%	4.20%
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	2.95%	2.63%	2.77%
Expected return on plan assets, net of administration expenses	3.64%	3.34%	3.36%
Other | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	1.00%	1.00%	1.00%
Other | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Rate of compensation increase	3.00%	3.00%	3.50%
Other | Pension Plan | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	1.89%	1.78%	1.85%
Expected return on plan assets, net of administration expenses	2.50%	1.70%	2.68%
Other | Pension Plan | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.88%	3.39%	3.81%
Expected return on plan assets, net of administration expenses	4.10%	4.85%	5.15%

Employee Benefits - Expected Re

Employee Benefits - Expected Return on Plan Assets Narrative (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets, net of administration expenses	7.05%	7.71%	7.88%

Employee Benefits - Schedule _2

Employee Benefits - Schedule of Fair Value of U.S Plan Assets (Details) - Pension Plan - U.S. - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	$ 2,066	$ 1,796	$ 1,958
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	77	130
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	195	384
Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	22	(14)
Equity investments, Pooled funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	583	285
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	128	111
Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	199	126
Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	3	2
Fixed Income Investments, Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	545	417
Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	133	78
Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	181	277
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	434	687
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	77	130
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	195	384
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments, Pooled funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	162	95
Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income Investments, Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	78
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	190	130
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	22	(14)
Significant Other Observable Inputs (Level 2) | Equity investments, Pooled funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	128	111
Significant Other Observable Inputs (Level 2) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	37	31
Significant Other Observable Inputs (Level 2) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	3	2
Significant Other Observable Inputs (Level 2) | Fixed Income Investments, Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Equity derivatives
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Equity investments, Pooled funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Government and agency bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Fixed Income Investments, Pooled Funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Real estate and REITS
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Alternative investments
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, plan assets	$ 0	$ 0

Employee Benefits - Schedule _3

Employee Benefits - Schedule of Fair Value of U.K. Plan Assets (Details) - Pension Plan - U.K. - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	$ 5,959	$ 5,225	$ 5,906
Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	81	96
Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	119	212
Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	(1,205)	(949)
Corporate bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		367
Government and agency bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	2,667	2,079
Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	1,849	1,688
Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	1,486	889
Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	180	149
Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	782	694
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	2,748	2,175
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	81	96
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Government and agency bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	2,667	2,079
Quoted Prices in Active Markets for Identical Assets (Level 1) | Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	(1,205)	(582)
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	(1,205)	(949)
Significant Other Observable Inputs (Level 2) | Corporate bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		367
Significant Other Observable Inputs (Level 2) | Government and agency bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	1,849	1,688	$ 1,909
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Corporate bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		0
Significant Unobservable Inputs (Level 3) | Government and agency bonds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Annuities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	1,849	1,688
Significant Unobservable Inputs (Level 3) | Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	$ 0	$ 0

Employee Benefits - Schedule _4

Employee Benefits - Schedule of changes in Level 3 fair value for U.K. Pension Plans (Details) - Pension Plan - U.K. - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Actual return on plan assets:
Balance at the beginning of the period	$ 5,225	$ 5,906
Foreign exchange	159	(317)
Balance at the end of the period	5,959	5,225
Significant Unobservable Inputs (Level 3)
Actual return on plan assets:
Balance at the beginning of the period	1,688	1,909
Relating to assets still held at the end of the year	113	(122)
Purchases, sales and settlements—net		7
Foreign exchange	48	(106)
Balance at the end of the period	$ 1,849	$ 1,688

Employee Benefits - Schedule _5

Employee Benefits - Schedule of Fair Value of Other Plan Assets (Details) - Other - Pension Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	$ 1,303	$ 1,155	$ 1,256
Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	5	10
Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	323	281
Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		9
Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	907	782
Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	62	63
Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	6	10
Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	5	10
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	5	10
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	9
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		9
Significant Other Observable Inputs (Level 2) | Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Other Observable Inputs (Level 2) | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Equity investments, Pooled funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Derivatives
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets		0
Significant Unobservable Inputs (Level 3) | Fixed Income Investments, Pooled Funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Alternative investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	0	0
Significant Unobservable Inputs (Level 3) | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, plan assets	$ 0	$ 0

Employee Benefits - Investment

Employee Benefits - Investment Policy and Strategy Narrative (Details)	Dec. 31, 2019
Foreign Plan | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual allocation percentage	8.00%
Foreign Plan | Fixed income investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual allocation percentage	82.00%
Foreign Plan | Other Investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actual allocation percentage	10.00%
Pension Plan | U.S. | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percentage	50.00%
Pension Plan | U.S. | Fixed income investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percentage	29.00%
Pension Plan | U.S. | Other Investments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percentage	21.00%

Employee Benefits - Cash Flows

Employee Benefits - Cash Flows Narrative (Details) $ in Millions	Dec. 31, 2019USD ($)
Other Post-Retirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected amortization of prior service credit, next fiscal year	$ 0.2
Expected amortization of net gain, next fiscal year	0.8
Expected employer contributions during next fiscal year	4
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected amortization of net gain, next fiscal year	(68)
Expected employer contributions during next fiscal year	99
Foreign Plan | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected amortization of net gain, next fiscal year	(44)
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected employer contributions during next fiscal year	5
Other | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected employer contributions during next fiscal year	$ 19

Employee Benefits - Estimated F

Employee Benefits - Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2019USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2024	$ 5
U.S. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2020	186
2021	190
2022	192
2023	189
2024	181
2025 - 2029	887
U.K. | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2020	164
2021	154
2022	160
2023	167
2024	172
2025 - 2029	913
Other | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2020	43
2021	44
2022	46
2023	46
2024	47
2025 - 2029	$ 252

Employee Benefits - Overview of

Employee Benefits - Overview of the accumulated benefit obligation, fair value of plan assets, funded status and net amount recognized for U.S. and Canadian Other Post-Retirement Benefits (Details) - Other Post-Retirement Benefits - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated projected benefit obligation	$ 103	$ 91
Fair value of plan assets	16	14
Funded status	(87)	(77)
Unrecognized prior-service credit	(1)	(1)
Unrecognized loss	3	(6)
Net amount recognized	$ (85)	$ (84)

Employee Benefits - Schedule _6

Employee Benefits - Schedule of Other information related to Company's other post-retirement plan's (Details) - Other Post-Retirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost recognized (millions)	$ 3	$ 3	$ 1
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted-average discount rate used to determine future benefit obligations	2.93%	3.91%	3.32%
Weighted-average discount rate used to determine net periodic benefit costs	3.91%	3.32%	3.71%
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted-average discount rate used to determine future benefit obligations	3.25%	4.26%	3.64%
Weighted-average discount rate used to determine net periodic benefit costs	4.26%	3.64%	4.15%

Employee Benefits - U.S. and Ca

Employee Benefits - U.S. and Canadian Other Post-Retirement Benefits Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected future benefit payments 2024	$ 5
Other Post-Retirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unrecognized prior-service cost	1	$ 1
Unrecognized loss	3	$ (6)
Amounts in Accumulated other comprehensive income (loss) expected to be recognized as components of net periodic benefit cost in next fiscal year	0.8
Expected employer contributions during next fiscal year	4
Expected future benefit payments 2020	5
Expected future benefit payments 2021	5
Expected future benefit payments 2022	5
Expected future benefit payments 2023	5
Expected future benefit payments 2025-2029	25
Effect of one percentage point increase on accumulated postretirement benefit obligation	3
Effect of one percentage point decrease on accumulated postretirement benefit obligation	2
Effect of one percentage point increase on service cost and interest cost components of net periodic benefit cost	0.1
Effect of one percentage point decrease on service cost and interest cost components of net periodic benefit cost	$ 0.1
Maximum percentage of liability for future plan cost increases for pre-65 and medical supplement plan coverage	5.00%
Maximum percentage of medical supplement plan into future periods	5.00%
Other Post-Retirement Benefits | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Maximum percentage of liability for future plan cost increases for pre-65 and medical supplement plan coverage	7.00%
Other Post-Retirement Benefits | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Maximum percentage of liability for future plan cost increases for pre-65 and medical supplement plan coverage	4.00%

Share-based Compensation Plan_2

Share-based Compensation Plans - Share-based compensation expense recognized in continuing operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	$ 317	$ 338	$ 319
Tax benefit	66	74	73
Share-based compensation expense, net of tax	251	264	246
Employee share purchase plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	9	9	10
Restricted share units (“RSUs”)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	198	186	182
Performance share awards (“PSAs”)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	$ 110	$ 143	$ 127

Share-based Compensation Plan_3

Share-based Compensation Plans - Restricted Share Units Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining amortization period (in years)	2 years 1 month 6 days
Restricted share units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of RSUs vested during the period	$ 187	$ 189	$ 197
Unamortized deferred compensation	$ 349
Restricted share units (RSUs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Restricted share units (RSUs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	5 years

Share-based Compensation Plan_4

Share-based Compensation Plans - Summary of the status of the Company's RSUs (Details) - Restricted share units (“RSUs”) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Non-vested share awards
Non-vested at beginning of period (in shares)	4,208	4,849	6,195
Granted (in shares)	1,306	1,500	1,700
Vested (in shares)	(1,661)	(1,943)	(2,407)
Forfeited (in shares)	(219)	(198)	(639)
Non-vested at end of period (in shares)	3,634	4,208	4,849
Fair Value at Date of Grant
Non-vested at beginning of period (in dollars per share)	$ 120	$ 104	$ 89
Granted (in dollars per share)	175	141	123
Vested (in dollars per share)	113	97	82
Forfeited (in dollars per share)	131	114	93
Non-vested at end of period (in dollars per share)	$ 143	$ 120	$ 104

Share-based Compensation Plan_5

Share-based Compensation Plans - Performance Share Awards Narrative (Details) - Performance-based Awards - shares shares in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
PSA, vesting conditions period (in years)	3 years
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
PSA, actual shares issued, percent	0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
PSA, actual shares issued, percent	200.00%
Leadership Performance Plan Cycle 2014-2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (in shares)	0.7
Leadership Performance Plan Cycle 2013-2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (in shares)		1
Leadership Performance Plan Cycle 2012-2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued (in shares)			0.9

Share-based Compensation Plan_6

Share-based Compensation Plans - Schedule of Performance-based plans (Details) - Performance-based Awards - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Target PSAs granted (in shares)	467	564	548
Fair value (in dollars per share)	$ 165	$ 134	$ 114
Number of shares that would be issued based on current performance levels (in shares)	453	818	1,057
Unamortized expense, based on current performance levels	$ 50	$ 34	$ 0

Derivatives and Hedging - Narra

Derivatives and Hedging - Narrative (Details) € in Millions, $ in Millions	Dec. 31, 2019USD ($)	Dec. 31, 2019EUR (€)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2018EUR (€)	Dec. 31, 2017USD ($)
Derivative [Line Items]
Estimated pretax losses currently included within Accumulated Other Comprehensive Loss that will be reclassified to earnings in next twelve months			$ 11
Financial instruments			$ (34)	$ (49)		$ 2
Not accounted for as hedges
Derivative [Line Items]
Maximum length of time hedged in foreign currency undesignated hedge (in years)			1 year
Gross foreign exchange contracts | Not accounted for as hedges
Derivative [Line Items]
Derivative term of contract (in days)			30 days
Financial instruments			$ (18)	(27)		$ 7
Cash Flow Hedging
Derivative [Line Items]
Foreign currency exposures, maximum average hedging period (in years)			2 years
Net Investment Hedging
Derivative [Line Items]
Outstanding Euro-denominated commercial paper	$ 112	€ 101		250	€ 220
Unrealized gain recognized in AOCI			$ 29	$ 21

Derivatives and Hedging - Notio

Derivatives and Hedging - Notional and fair values of derivative instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Other Current Assets
Derivative [Line Items]
Derivative assets	$ 7	$ 3
Other Noncurrent Assets
Derivative [Line Items]
Derivative assets	11	15
Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	5
Other Noncurrent Liabilities
Derivative [Line Items]
Derivative liabilities		3
Accounted for as hedges | Gross foreign exchange contracts
Derivative [Line Items]
Derivative, Notional Amount	579	646
Derivative assets	16	17
Derivative liabilities	1	2
Not accounted for as hedges
Derivative [Line Items]
Derivative, Notional Amount	876	915
Derivative assets	18	18
Derivative liabilities	1	8
Not accounted for as hedges | Gross foreign exchange contracts
Derivative [Line Items]
Derivative, Notional Amount	297	269
Derivative assets	2	1
Derivative liabilities	$ 0	$ 6
Derivative term of contract (in days)	30 days

Derivatives and Hedging - Sched

Derivatives and Hedging - Schedule of amounts of derivative gains (losses) recognized in the Consolidated Financial Statements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
(Loss) Gain recognized in Accumulated other comprehensive loss	$ (9)
(Loss) Gain recognized in Accumulated other comprehensive loss		$ (18)	$ 18
Gain (loss) reclassified from accumulated other comprehensive loss, after adoption of ASU 2017-02	(14)
Gain (loss) reclassified from accumulated other comprehensive loss into income (effective portion)		(11)	(1)
Total revenue
Derivative [Line Items]
Gain (loss) reclassified from accumulated other comprehensive loss, after adoption of ASU 2017-02	(12)
Gain (loss) reclassified from accumulated other comprehensive loss into income (effective portion)		0	0
Compensation and benefits
Derivative [Line Items]
Gain (loss) reclassified from accumulated other comprehensive loss, after adoption of ASU 2017-02	(1)
Gain (loss) reclassified from accumulated other comprehensive loss into income (effective portion)		1	14
Other general expense
Derivative [Line Items]
Gain (loss) reclassified from accumulated other comprehensive loss, after adoption of ASU 2017-02	0
Gain (loss) reclassified from accumulated other comprehensive loss into income (effective portion)		(2)	(5)
Interest expense
Derivative [Line Items]
Gain (loss) reclassified from accumulated other comprehensive loss, after adoption of ASU 2017-02	(1)
Gain (loss) reclassified from accumulated other comprehensive loss into income (effective portion)		(2)	(1)
Other income (expense)
Derivative [Line Items]
Gain (loss) reclassified from accumulated other comprehensive loss, after adoption of ASU 2017-02	$ 0
Gain (loss) reclassified from accumulated other comprehensive loss into income (effective portion)		$ (8)	$ (9)

Fair Value Measurements and F_3

Fair Value Measurements and Financial Instruments - Schedule of assets and liabilities that are measured at fair value on a recurring basis (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Money market funds
Assets
Money market funds	$ 2,007	$ 1,759
Government bonds
Assets
Other investments	1	1
Equity investments
Assets
Other investments	1	2
Gross foreign exchange contracts
Assets
Gross foreign exchange contracts	21	21
Liabilities
Gross foreign exchange contracts	4	12
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets
Money market funds	2,007	1,759
Quoted Prices in Active Markets for Identical Assets (Level 1) | Government bonds
Assets
Other investments	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity investments
Assets
Other investments	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Gross foreign exchange contracts
Assets
Gross foreign exchange contracts	0	0
Liabilities
Gross foreign exchange contracts	0	0
Significant Other Observable Inputs (Level 2) | Money market funds
Assets
Money market funds	0	0
Significant Other Observable Inputs (Level 2) | Government bonds
Assets
Other investments	1	1
Significant Other Observable Inputs (Level 2) | Equity investments
Assets
Other investments	1	2
Significant Other Observable Inputs (Level 2) | Gross foreign exchange contracts
Assets
Gross foreign exchange contracts	21	21
Liabilities
Gross foreign exchange contracts	4	12
Significant Unobservable Inputs (Level 3) | Money market funds
Assets
Money market funds	0	0
Significant Unobservable Inputs (Level 3) | Government bonds
Assets
Other investments	0	0
Significant Unobservable Inputs (Level 3) | Equity investments
Assets
Other investments	0	0
Significant Unobservable Inputs (Level 3) | Gross foreign exchange contracts
Assets
Gross foreign exchange contracts	0	0
Liabilities
Gross foreign exchange contracts	$ 0	$ 0

Fair Value Measurements and F_4

Fair Value Measurements and Financial Instruments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Unrealized gain (loss) related to assets and liabilities measured at fair value using unobservable inputs	$ 0	$ 0	$ 0

Fair Value Measurements and F_5

Fair Value Measurements and Financial Instruments - Schedule of financial instruments where the carrying amounts and fair values differ (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair value of financial instrument
Current portion of long-term debt	$ 600	$ 0
Fair value current portion of long-term debt	614	0
Long-term debt	6,627	5,993
Fair value of long term debt	$ 7,442	$ 6,159

Claims, Lawsuits, and Other Con

Claims, Lawsuits, and Other Contingencies - Legal Narrative (Details) £ in Millions, $ in Millions	Dec. 31, 2019USD ($)	Dec. 31, 2019NZD ($)	Sep. 06, 2018USD ($)	Jun. 29, 2015NZD ($)	Dec. 31, 2016USD ($)	Oct. 20, 2016GBP (£)	Apr. 30, 2014GBP (£)
Lyttleton Port Company Limited
Loss Contingencies [Line Items]
Value of damages sought	$ 123,000,000			$ 184
Pending Litigation | Christchurch City Council
Loss Contingencies [Line Items]
Value of damages sought	352,000,000	$ 528
Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	0
Minimum | Pilkington North America, Inc.
Loss Contingencies [Line Items]
Value of damages sought			$ 45,000,000
Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	100,000,000
Maximum | Pilkington North America, Inc.
Loss Contingencies [Line Items]
Value of damages sought			85,000,000
Damages awarded			$ 15,000,000
Aviation and Aerospace Broking Industry
Loss Contingencies [Line Items]
Total revenue					$ 100,000,000
Potential Claim for Pension Advisory Services
Loss Contingencies [Line Items]
Estimate of possible loss	58,000,000						£ 45
Potential Claim for Pension Advisory Services | Trustees of Gleeds Pension Fund 2016
Loss Contingencies [Line Items]
Estimate of possible loss	$ 91,000,000					£ 70

Claims, Lawsuits, and Other C_2

Claims, Lawsuits, and Other Contingencies - Guarantees and Indemnifications Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Loss Contingencies [Line Items]
Maximum potential funding under commitments	$ 110	$ 103
Discontinued Operations, Disposed of by Sale | Tempo Business | Property Lease Guarantee
Loss Contingencies [Line Items]
Maximum potential funding under commitments	70
Guarantor obligations, current carrying value	12
Discontinued Operations, Disposed of by Sale | Tempo Business | Performance Guarantee
Loss Contingencies [Line Items]
Maximum potential funding under commitments	151
Guarantor obligations, current carrying value	$ 1

Claims, Lawsuits, and Other C_3

Claims, Lawsuits, and Other Contingencies - Letters of Credit Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Letters of credit outstanding	$ 73	$ 83

Claims, Lawsuits, and Other C_4

Claims, Lawsuits, and Other Contingencies - Premium Payments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Maximum potential funding under commitments	$ 110	$ 103

Segment Information - Narrative

Segment Information - Narrative (Details)	12 Months Ended
	Dec. 31, 2019segmentmetricrevenue_line
Segment Reporting [Abstract]
Number of reportable segments | segment	1
Number of revenue lines | revenue_line	5
Number of performance metrics measured | metric	4

Segment Information - Schedule

Segment Information - Schedule of consolidated non-current assets by geographic area (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, including operating lease ROU assets	$ 1,550
Fixed assets, net	621	$ 588
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, including operating lease ROU assets	686
Fixed assets, net		288
Americas other than U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, including operating lease ROU assets	145
Fixed assets, net		44
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, including operating lease ROU assets	225
Fixed assets, net		58
Europe, Middle East, & Africa
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, including operating lease ROU assets	319
Fixed assets, net		101
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets, including operating lease ROU assets	$ 175
Fixed assets, net		$ 97

Guarantee of Registered Secur_3

Guarantee of Registered Securities - Narrative (Details)	Dec. 31, 2019	Dec. 03, 2018
5.00% Senior Notes due September 2020
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	5.00%
8.205% Junior Subordinated Deferrable Interest Debentures Percent Due 2027
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	8.205%
6.25% Senior Notes due September 2040
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	6.25%
3.75% Senior Notes due May 2029
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	3.75%
4.250% Notes Due 2042
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	4.25%
4.45% Senior Notes due May 2043
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	4.45%
4.00% Senior Notes due November 2023
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	4.00%
2.875% Senior Notes due May 2026 (EUR 500M)
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	2.875%
3.50% Senior Notes due June 2024
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	3.50%
4.60% Senior Notes due June 2044
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	4.60%
4.50% Senior Notes due December 2028 | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	4.50%	4.50%
2.20% Senior Notes due November 2022 | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	2.20%
4.75% Senior Notes due May 2045
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	4.75%
4.75% Senior Notes due May 2045 | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	4.75%
2.80% Senior Notes due March 2021
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	2.80%
2.80% Senior Notes due March 2021 | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	2.80%
3.875% Senior Notes due December 2025
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	3.875%
3.875% Senior Notes due December 2025 | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Interest rate on debt	3.875%
Aon plc
Condensed Financial Statements, Captions [Line Items]
Subsidiary guarantors percentage owned	100.00%

Guarantee of Registered Secur_4

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 plc
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 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									0	0	4
Interest expense									(17)	(27)	(15)
Intercompany interest income (expense)									443	499	529
Intercompany other income (expense)									177	308	103
Other income (expense)									51	3	(93)
Income from continuing operations before income taxes									2,929	2,494	1,801
Income tax expense (benefit)									491	398	403
Net income from continuing operations									2,438	2,096	1,398
Net income (loss) from discontinued operations									(1)	74	828
Net income (loss) before equity in earnings of subsidiaries									2,437	2,170	2,226
Equity in earnings of subsidiaries									409	134	247
Net income									2,846	2,304	2,473
Less: Net income attributable to noncontrolling interests									41	40	37
Net income attributable to Aon shareholders									2,805	2,264	2,436
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									(31)	(53)	(29)
Interest expense									31	53	29
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									(3,214)	(2,398)	(2,683)
Net income									(3,222)	(2,419)	(2,665)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									$ (3,222)	$ (2,419)	$ (2,665)

Guarantee of Registered Secur_5

Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive 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
Condensed Financial Statements, Captions [Line Items]
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
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									3	11	12
Foreign currency translation adjustments									14	(444)	390
Postretirement benefit obligation									(141)	17	19
Total other comprehensive income (loss)									(124)	(416)	421
Equity in other comprehensive income (loss) of subsidiaries, net of tax									0	0	0
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	(4)	5
Total other comprehensive income (loss) attributable to Aon shareholders									(124)	(412)	416
Comprehensive income attributable to Aon shareholders									1,408	722	1,642
Aon plc
Condensed Financial Statements, Captions [Line Items]
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
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									0	0	0
Foreign currency translation adjustments									0	0	0
Postretirement benefit obligation									0	0	0
Total other comprehensive income (loss)									0	0	0
Equity in other comprehensive income (loss) of subsidiaries, net of tax									(132)	(433)	434
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									(132)	(433)	434
Comprehensive income attributable to Aon shareholders									1,408	722	1,642
Aon Corporation
Condensed Financial Statements, Captions [Line Items]
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 comprehensive income (loss), net of tax:
Change in fair value of financial instruments									3	0	3
Foreign currency translation adjustments									0	0	0
Postretirement benefit obligation									(44)	(2)	(101)
Total other comprehensive income (loss)									(41)	(2)	(98)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									(131)	(415)	515
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									(172)	(417)	417
Comprehensive income attributable to Aon shareholders									237	(283)	664
Other Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income									2,846	2,304	2,473
Less: Net income attributable to noncontrolling interests									41	40	37
Net income attributable to Aon shareholders									2,805	2,264	2,436
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									0	11	9
Foreign currency translation adjustments									6	(465)	408
Postretirement benefit obligation									(97)	19	120
Total other comprehensive income (loss)									(91)	(435)	537
Equity in other comprehensive income (loss) of subsidiaries, net of tax									(172)	(417)	417
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	(4)	5
Total other comprehensive income (loss) attributable to Aon shareholders									(263)	(848)	949
Comprehensive income attributable to Aon shareholders									2,542	1,416	3,385
Consolidating Adjustments
Condensed Financial Statements, Captions [Line Items]
Net income									(3,222)	(2,419)	(2,665)
Less: Net income attributable to noncontrolling interests									0	0	0
Net income attributable to Aon shareholders									(3,222)	(2,419)	(2,665)
Other comprehensive income (loss), net of tax:
Change in fair value of financial instruments									0	0	0
Foreign currency translation adjustments									8	21	(18)
Postretirement benefit obligation									0	0	0
Total other comprehensive income (loss)									8	21	(18)
Equity in other comprehensive income (loss) of subsidiaries, net of tax									435	1,265	(1,366)
Less: Other comprehensive income (loss) attributable to noncontrolling interests									0	0	0
Total other comprehensive income (loss) attributable to Aon shareholders									443	1,286	(1,384)
Comprehensive income attributable to Aon shareholders									$ (2,779)	$ (1,133)	$ (4,049)

Guarantee of Registered Secur_6

Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Total current assets
Cash and cash equivalents	$ 790	$ 656
Short-term investments	138	172
Receivables, net	3,112	2,760
Fiduciary assets	11,834	10,166
Intercompany receivables	0	0
Other current assets	602	618
Total current assets	16,476	14,372
Goodwill	8,165	8,171	$ 8,358
Intangible assets, net	783	1,149
Operating lease right-of-use assets	929
Fixed assets, net	621	588
Deferred tax assets	645	561
Deferred tax assets	0	0
Prepaid pension	1,216	1,133
Other non-current assets	570	448
Investment in subsidiary	0	0
Total assets	29,405	26,422
Current liabilities
Accounts payable and accrued liabilities	1,939	1,943
Short-term debt and current portion of long-term debt	712	251
Fiduciary liabilities	11,834	10,166
Intercompany payables	0	0
Other current liabilities	1,086	936
Total current liabilities	15,571	13,296
Long-term debt	6,627	5,993
Non-current operating lease liabilities	944
Deferred tax liabilities	199	181
Pension, other postretirement, and other post-employment liabilities	1,738	1,636
Intercompany payables	0	0
Other non-current liabilities	877	1,097
Total liabilities	25,956	22,203
Equity [Abstract]
Total Aon shareholders' equity	3,375	4,151
Noncontrolling interests	74	68
Total equity	3,449	4,219	$ 4,648	$ 5,581
Total liabilities and equity	29,405	26,422
Aon plc
Total current assets
Cash and cash equivalents	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	246	191
Other current assets	0	0
Total current assets	246	191
Goodwill	0	0
Intangible assets, net	0	0
Operating lease right-of-use assets	0
Fixed assets, net	0	0
Deferred tax assets	89	94
Deferred tax assets	868	403
Prepaid pension	0	0
Other non-current assets	0	1
Investment in subsidiary	8,899	8,433
Total assets	10,102	9,122
Current liabilities
Accounts payable and accrued liabilities	2,157	274
Short-term debt and current portion of long-term debt	112	250
Fiduciary liabilities	0	0
Intercompany payables	234	213
Other current liabilities	0	0
Total current liabilities	2,503	737
Long-term debt	4,223	4,231
Non-current operating lease liabilities	0
Deferred tax liabilities	0	0
Pension, other postretirement, and other post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	1	3
Total liabilities	6,727	4,971
Equity [Abstract]
Total Aon shareholders' equity	3,375	4,151
Noncontrolling interests	0	0
Total equity	3,375	4,151
Total liabilities and equity	10,102	9,122
Aon Corporation
Total current assets
Cash and cash equivalents	2,271	862
Short-term investments	28	56
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	1,214	897
Other current assets	7	16
Total current assets	3,520	1,831
Goodwill	0	0
Intangible assets, net	0	0
Operating lease right-of-use assets	110
Fixed assets, net	0	0
Deferred tax assets	577	467
Deferred tax assets	261	261
Prepaid pension	7	5
Other non-current assets	32	30
Investment in subsidiary	19,470	19,031
Total assets	23,977	21,625
Current liabilities
Accounts payable and accrued liabilities	56	70
Short-term debt and current portion of long-term debt	600	0
Fiduciary liabilities	0	0
Intercompany payables	12,978	11,875
Other current liabilities	80	69
Total current liabilities	13,714	12,014
Long-term debt	2,404	1,762
Non-current operating lease liabilities	143
Deferred tax liabilities	0	0
Pension, other postretirement, and other post-employment liabilities	1,348	1,275
Intercompany payables	7,212	7,390
Other non-current liabilities	113	167
Total liabilities	24,934	22,608
Equity [Abstract]
Total Aon shareholders' equity	(957)	(983)
Noncontrolling interests	0	0
Total equity	(957)	(983)
Total liabilities and equity	23,977	21,625
Other Non-Guarantor Subsidiaries
Total current assets
Cash and cash equivalents	468	575
Short-term investments	110	116
Receivables, net	3,112	2,760
Fiduciary assets	11,834	10,166
Intercompany receivables	12,799	11,634
Other current assets	595	602
Total current assets	28,918	25,853
Goodwill	8,165	8,171
Intangible assets, net	783	1,149
Operating lease right-of-use assets	819
Fixed assets, net	621	588
Deferred tax assets	169	144
Deferred tax assets	7,046	7,225
Prepaid pension	1,209	1,128
Other non-current assets	538	417
Investment in subsidiary	(957)	(983)
Total assets	47,311	43,692
Current liabilities
Accounts payable and accrued liabilities	1,675	2,380
Short-term debt and current portion of long-term debt	0	1
Fiduciary liabilities	11,834	10,166
Intercompany payables	1,047	634
Other current liabilities	1,006	867
Total current liabilities	15,562	14,048
Long-term debt	0	0
Non-current operating lease liabilities	801
Deferred tax liabilities	389	325
Pension, other postretirement, and other post-employment liabilities	390	361
Intercompany payables	963	499
Other non-current liabilities	763	927
Total liabilities	18,868	16,160
Equity [Abstract]
Total Aon shareholders' equity	28,369	27,464
Noncontrolling interests	74	68
Total equity	28,443	27,532
Total liabilities and equity	47,311	43,692
Consolidating Adjustments
Total current assets
Cash and cash equivalents	(1,949)	(781)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(14,259)	(12,722)
Other current assets	0	0
Total current assets	(16,208)	(13,503)
Goodwill	0	0
Intangible assets, net	0	0
Operating lease right-of-use assets	0
Fixed assets, net	0	0
Deferred tax assets	(190)	(144)
Deferred tax assets	(8,175)	(7,889)
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	(27,412)	(26,481)
Total assets	(51,985)	(48,017)
Current liabilities
Accounts payable and accrued liabilities	(1,949)	(781)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Intercompany payables	(14,259)	(12,722)
Other current liabilities	0	0
Total current liabilities	(16,208)	(13,503)
Long-term debt	0	0
Non-current operating lease liabilities	0
Deferred tax liabilities	(190)	(144)
Pension, other postretirement, and other post-employment liabilities	0	0
Intercompany payables	(8,175)	(7,889)
Other non-current liabilities	0	0
Total liabilities	(24,573)	(21,536)
Equity [Abstract]
Total Aon shareholders' equity	(27,412)	(26,481)
Noncontrolling interests	0	0
Total equity	(27,412)	(26,481)
Total liabilities and equity	$ (51,985)	$ (48,017)

Guarantee of Registered Secur_7

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 plc
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
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	1
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
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	2,408	3,608	2,010
Cash provided by operating activities - discontinued operations	0	0	65
Cash provided by (used for) operating activities	2,408	3,608	2,075
Cash flows from investing activities
Proceeds from investments	189	955	582
Payments for investments	(52)	(33)	(576)
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	(68)	663	3,023
Cash used for investing activities - discontinued operations	0	0	(19)
Cash provided by (used for) investing activities	(68)	663	3,004
Cash flows from financing activities
Share repurchase	0	0	0
Advances from (to) affiliates	(2,365)	(4,041)	(4,975)
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	(2,468)	(4,371)	(5,015)
Cash used for financing activities - discontinued operations	0		0
Cash used for financing activities	(2,468)	(4,371)	(5,015)
Effect of exchange rates on cash and cash equivalents	21	(118)	69
Net increase (decrease) in cash and cash equivalents	(107)	(218)	133
Cash and cash equivalents at beginning of period	575	793	660
Cash and cash equivalents at end of period	468	575	793
Consolidating Adjustments
Cash flows from operating activities
Cash provided by (used for) operating activities - continuing operations	(713)	(3,500)	(4,631)
Cash provided by operating activities - discontinued operations	0	0	0
Cash provided by (used for) operating activities	(713)	(3,500)	(4,631)
Cash flows from investing activities
Proceeds from investments	(157)	(908)	(1,325)
Payments for investments	1	13	802
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	(156)	(895)	(523)
Cash used for investing activities - discontinued operations	0	0	0
Cash provided by (used for) investing activities	(156)	(895)	(523)
Cash flows from financing activities
Share repurchase	0	0	0
Advances from (to) affiliates	(299)	6,176	4,454
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	(299)	6,176	4,454
Cash used for financing activities - discontinued operations	0		0
Cash used for financing activities	(299)	6,176	4,454
Effect of exchange rates on cash and cash equivalents	0	0	0
Net increase (decrease) in cash and cash equivalents	(1,168)	1,781	(700)
Cash and cash equivalents at beginning of period	(781)	(2,562)	(1,862)
Cash and cash equivalents at end of period	$ (1,949)	$ (781)	$ (2,562)

Quarterly Financial Data (Una_3

Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 2,885	$ 2,379	$ 2,606	$ 3,143	$ 2,770	$ 2,349	$ 2,561	$ 3,090	$ 11,013	$ 10,770	$ 9,998
Operating income	524	360	413	872	499	262	(16)	799	2,169	1,544	1,065
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	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
Basic net income per share attributable to Aon shareholders
Basic net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	$ 1.59	$ 0.94	$ 1.15	$ 2.72	$ 1.14	$ 0.61	$ 0.19	$ 2.37	$ 6.42	$ 4.32	$ 1.54
Basic net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	0	0	0	0	0.28	(0.01)	0.01	0.02	0	0.30	3.20
Basic net income per share attributable to Aon shareholders (in dollars per share)	1.59	0.94	1.15	2.72	1.42	0.60	0.20	2.39	6.42	4.62	4.74
Diluted net income per share attributable to Aon shareholders
Diluted net income per share attributable to Aon shareholders, continuing operations (in dollars per share)	1.58	0.93	1.14	2.70	1.13	0.61	0.19	2.35	6.37	4.29	1.53
Diluted net income per share attributable to Aon shareholders, discontinued operations (in dollars per share)	0	0	0	0	0.28	(0.01)	0	0.02	0	0.30	3.17
Diluted net income per share attributable to Aon shareholders (in dollars per share)	$ 1.58	$ 0.93	$ 1.14	$ 2.70	$ 1.41	$ 0.60	$ 0.19	$ 2.37	$ 6.37	$ 4.59	$ 4.70

Uncategorized Items - aonplc201

Label	Element	Value
Common Stock Including Additional Paid in Capital [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	$ 5,777,000,000
Noncontrolling Interest [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	65,000,000
Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	2,795,000,000
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 493,000,000